build: remove obsolete TF-Fuzz CMake options

Remove the -DTFM_FUZZER_TOOL_TESTS option. Test suites generated by
TF-Fuzz do not work with the current regression test system, so this
option no longer serves any purpose.

Signed-off-by: Nik Dewally <Nik.Dewally@arm.com>
Change-Id: I8714a8bd8d7b1eaad613217af6fd03266dac2ee6