[S390] remove superfluous TIF_USEDFPU bit

The TIF_USEDFPU bit is always 0 for s390 and it is not tested anywhere.
Remove the bit. At the same time remove the calls to clear_used_math()
as well. The PF_USED_MATH bit is never set for s390 either.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>