python: Don't require icetea

icetea is used for running icetea integration tests with `mbed-cli test
--icetea`, but not otherwise needed. icetea was intended for deprecation
with Mbed OS 6.0. Remove icetea from the list of default dependencies,
as the version of prettytable it depends on causes a Python dependency
conflict with other libraries' dependencies on prettytable, preventing
icetea from being installed to the same Python environment as other Mbed
Python tools.

requirements.txt

@@ -18,7 +18,6 @@ mbed-greentea>=0.2.24,<2.0
 beautifulsoup4>=4,<=4.6.3
 pyelftools>=0.24,<=0.25
 manifest-tool==1.5.2
-icetea>=1.2.1,<1.3
 pycryptodome>=3.9.3,<4
 pyusb>=1.0.0,<2.0.0
 hidapi>=0.7.99,<0.8.0;platform_system!="Linux"