Remove commented-out code from gcore.c

Remove commented-out code from gcore.c

I found some code in gcore.c that has been commented out since

d3420b2fce5e (Mark Kettenis     2003-09-04 166) #if 1	/* See if this even matters...  */

This patch deletes this entire function, because the body was reduced
to just "return 0".

gdb/ChangeLog
2020-10-29  Tom Tromey  <tom@tromey.com>

	* gcore.c (default_gcore_mach): Remove.
	(create_gcore_bfd): Update.