Remove FP16 and BF16 requirement to build and run tests

* FP16 and BF16 classes are implemented in assembly so the rest
  of the test framework doesn't need to be compiled with FP16
  and BF16 support anymore. It allows the test to be run
  on system with base architecture.
* Remove unnecessary feature guard in kernel header file.
  The user of our API must not need to compile their code
  with BF16 support.

Signed-off-by: Viet-Hoa Do <viet-hoa.do@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>