Remove XBee support

Even though it hasn't been supported forever, the files where still
being compiled

QGCExternalLibs.pri

@@ -65,103 +65,6 @@ DEPENDPATH += libs/qwt
 INCLUDEPATH += libs/qwt
 }
 
-#
-# [OPTIONAL] XBee wireless support. This is not necessary for basic serial/UART communications.
-# It's only required for speaking directly to the Xbee using their proprietary API.
-# Unsupported on Mac.
-# Installation on Windows is unnecessary, as we just link to our included .dlls directly.
-# Installing on Linux involves running `make;sudo make install` in `libs/thirdParty/libxbee`
-# Uninstalling from Linux can be done with `sudo make uninstall`.
-#
-XBEE_DEPENDENT_HEADERS += \
-    src/comm/XbeeLinkInterface.h \
-    src/comm/XbeeLink.h \
-    src/comm/HexSpinBox.h \
-    src/ui/XbeeConfigurationWindow.h \
-    src/comm/CallConv.h
-XBEE_DEPENDENT_SOURCES += \
-    src/comm/XbeeLink.cpp \
-    src/comm/HexSpinBox.cpp \
-    src/ui/XbeeConfigurationWindow.cpp
-XBEE_DEFINES = QGC_XBEE_ENABLED
-
-contains(DEFINES, DISABLE_XBEE) {
-    message("Skipping support for native XBee API (manual override from command line)")
-    DEFINES -= DISABLE_XBEE
-# Otherwise the user can still disable this feature in the user_config.pri file.
-} else:exists(user_config.pri):infile(user_config.pri, DEFINES, DISABLE_XBEE) {
-    message("Skipping support for native XBee API (manual override from user_config.pri)")
-} else:LinuxBuild {
-        linux-g++-64 {
-            message("Skipping support for XBee API (64-bit Linux builds not supported)")
-        } else:exists(/usr/include/xbee.h) {
-        message("Including support for XBee API")
-
-        HEADERS += $$XBEE_DEPENDENT_HEADERS
-        SOURCES += $$XBEE_DEPENDENT_SOURCES
-        DEFINES += $$XBEE_DEFINES
-        LIBS += -L/usr/lib -lxbee
-    } else {
-        warning("Skipping support for XBee API (missing libraries, see README)")
-    }
-} else:WindowsBuild {
-    message("Including support for XBee API")
-    HEADERS += $$XBEE_DEPENDENT_HEADERS
-    SOURCES += $$XBEE_DEPENDENT_SOURCES
-    DEFINES += $$XBEE_DEFINES
-    INCLUDEPATH += libs/thirdParty/libxbee
-        LIBS += -l$$BASEDIR/libs/thirdParty/libxbee/lib/libxbee
-} else {
-    message("Skipping support for XBee API (unsupported platform)")
-}
-
-#
-# [OPTIONAL] Opal RT-LAB Library. Provides integration with Opal-RT's RT-LAB simulator.
-#
-contains(DEFINES, DISABLE_RTLAB) {
-    message("Skipping support for RT-LAB (manual override from command line)")
-    DEFINES -= DISABLE_RTLAB
-# Otherwise the user can still disable this feature in the user_config.pri file.
-} else:exists(user_config.pri):infile(user_config.pri, DEFINES, DISABLE_RTLAB) {
-    message("Skipping support for RT-LAB (manual override from user_config.pri)")
-} else:WindowsBuild {
-    exists(src/lib/opalrt/OpalApi.h) : exists(C:/OPAL-RT/RT-LAB7.2.4/Common/bin) {
-        message("Including support for RT-LAB")
-
-        DEFINES += QGC_RTLAB_ENABLED
-
-        INCLUDEPATH +=
-            src/lib/opalrt
-            libs/lib/opal/include \
-
-        FORMS += src/ui/OpalLinkSettings.ui
-
-        HEADERS += \
-            src/comm/OpalRT.h \
-            src/comm/OpalLink.h \
-            src/comm/Parameter.h \
-            src/comm/QGCParamID.h \
-            src/comm/ParameterList.h \
-            src/ui/OpalLinkConfigurationWindow.h
-
-        SOURCES += \
-            src/comm/OpalRT.cc \
-            src/comm/OpalLink.cc \
-            src/comm/Parameter.cc \
-            src/comm/QGCParamID.cc \
-            src/comm/ParameterList.cc \
-            src/ui/OpalLinkConfigurationWindow.cc
-
-        LIBS += \
-            -LC:/OPAL-RT/RT-LAB7.2.4/Common/bin \
-            -lOpalApi
-    } else {
-        warning("Skipping support for RT-LAB (missing libraries, see README)")
-    }
-} else {
-    message("Skipping support for RT-LAB (unsupported platform)")
-}
-
 #
 # [REQUIRED] SDL dependency. Provides joystick/gamepad support.
 # The SDL is packaged with QGC for the Mac and Windows. Linux support requires installing the SDL