s390: Remove duplicate checks for cached gdbarch at init

s390: Remove duplicate checks for cached gdbarch at init

When initializing the gdbarch there is a check whether an appropriate
gdbarch already exists in the gdbarch_list.  Failing of some of the checks
would lead to a different target description.  However
gdbarch_list_lookup_by_info already checks for

	if (info->target_desc != arches->gdbarch->target_desc)
	  continue;

Remove these duplicate checks.

gdb/ChangeLog:

	* s390-linux-tdep.c (s390_gdbarch_init): Remove duplicate checks
	when looking for cached gdbarch and add comment for remaining.