s390/ptrace: simplify enable/disable single step

The user_enable_single_step() and user_disable_sindle_step() functions
are always called on the inferior, never for the currently active
process. Remove the unnecessary check for the current process and
the update_cr_regs() call from the enable/disable functions.

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