CMake: Remove unnecessary CMakeLists.txt

CMakeLists.txt file in /hal/tests/TESTS/mbed_hal_fpga_ci_test_shield
directory was non-functional as it used the greentea_add_test macro,
which expects a main.cpp in current directory, but no main.cpp exists
there. I checked with @rajkan01 who confirmed that this CMake file is
serving no purpose and is there erroneously. All tests in subdirectories
of this directory have their own CMakeLists.txt that successfully build
them.

Author: hazzlim