Remove some gdb/data-directory special cases

Remove some gdb/data-directory special cases

I found an odd special case for data-directory in gdb's Makefile.  I
don't see a reason to have this, so this removes it in favor of having
this code work in the most ordinary way for a subdirectory build.

gdb/ChangeLog
2021-06-01  Tom Tromey  <tromey@adacore.com>

	* Makefile.in (all-data-directory): Remove.
	(data-directory/Makefile): Remove.