s390/ptrace: remove leftover comment

The code which validates floating point control register contents was
reworked with commit 70264424 ("s390/fpu: get rid of test_fp_ctl()").

There is still a comment which refers to the old implementation - remove it
in order to avoid confusion.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>