hurd: remove unused variables

Fixes

../../gdb/gnu-nat.c:2554:7: error: unused variable ‘res’ [-Werror=unused-variable]
 2554 |   int res;
../../gdb/gnu-nat.c:2644:20: error: unused variable ‘old_address’ [-Werror=unused-variable]
 2644 |       vm_address_t old_address = region_address;

gdb/ChangeLog:

	* gnu-nat.c (gnu_xfer_auxv): Remove unused `res' variable.
	(gnu_nat_target::find_memory_regions): Remove unused
	`old_address' variable.