S390: gdbserver: Don't write guarded storage registers

On S390, the guarded storage register set is only valid if guarded storage
is active.  Reading/writing the register set yields errors if this is not
the case.  Then gdbserver emits warnings like these:

  Warning: ptrace(regsets_store_inferior_registers): No data available

Apart from confusing the user, this can also lead to test case failures
due to unexpected output.  To suppress this, make the guarded storage
regsets read-only for now.

gdb/gdbserver/ChangeLog:

	* linux-s390-low.c (s390_fill_gs): Remove function.
	(s390_fill_gsbc): Remove function.
	(s390_regsets): Set fill functions for the guarded storage regsets
	to NULL.