Revert "sim/Make.defs: add -fvisibility=hidden to CFLAGS"

This reverts commit 21cff9f.

It broke sim on macOS.
In the following backtrace, the host socket() should be used
instead of nuttx's.

(lldb) bt
* thread apache#1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x00000001000f5391 nuttx`socket(domain=1, type=1, protocol=0) at socket.c:192:12
    frame apache#1: 0x000000010012b682 nuttx`vpnkit_connect at up_vpnkit.c:75:8
    frame apache#2: 0x000000010012b60b nuttx`vpnkit_init at up_vpnkit.c:136:3
    frame apache#3: 0x0000000100034b56 nuttx`netdriver_init at up_netdriver.c:334:3
    frame apache#4: 0x0000000100033294 nuttx`up_initialize at up_initialize.c:260:3
    frame apache#5: 0x00000001000031e3 nuttx`nx_start at nx_start.c:701:3
    frame apache#6: 0x0000000100000b12 nuttx`main(argc=1, argv=0x00007ffeefbfd6c8, envp=0x00007ffeefbfd6d8) at up_head.c:96:7
    frame apache#7: 0x00007fff7831b3d5 libdyld.dylib`start + 1
    frame apache#8: 0x00007fff7831b3d5 libdyld.dylib`start + 1
(lldb)

boards/sim/sim/sim/scripts/Make.defs

@@ -41,16 +41,6 @@ ARCHPICFLAGS = -fpic
 ARCHWARNINGS = -Wall -Wstrict-prototypes -Wshadow -Wundef
 ARCHWARNINGSXX = -Wall -Wshadow -Wundef
 
-# Add -fvisibility=hidden
-# Because we don't want export nuttx's symbols to share libraries
-
-ARCHCPUFLAGS += -fvisibility=hidden
-ARCHCPUFLAGSXX += -fvisibility=hidden
-
-ifeq ($(CONFIG_HOST_MACOS),y)
-  LDLINKFLAGS += -keep_private_externs
-endif
-
 # Add -fno-common because macOS "ld -r" doesn't seem to pick objects
 # for common symbols.
 ARCHCPUFLAGS += -fno-common