Remove gdb workaround from readline/xfree.c

Remove gdb workaround from readline/xfree.c

There is a gdb-local patch to deal with interrupts during completion.
The original thread adding this patch is here:

https://sourceware.org/ml/gdb-patches/2011-06/msg00147.html

I believe readline now implements the approach suggested by
Chet Ramey:

https://sourceware.org/ml/gdb-patches/2011-06/msg00493.html

So, I believe this patch can be removed.

readline/ChangeLog.gdb
2018-10-07  Tom Tromey  <tom@tromey.com>

	* Makefile.in (xfree.o): Don't depend on readline.h.
	* xfree.c (xfree): Remove gdb workaround.
	* xmalloc.h (xfree): Remove #define.