Don't enable gdbtk in testsuite

Don't enable gdbtk in testsuite

When I skim configure.ac and Makefile.in in gdb/testsuite, I happen to
see that directory gdb.gdbtk is added to subdirs, however it doesn't
exist.  gdb/testsuite/gdb.gdbtk was removed by the patch below,

  [rfa] git repo fixup: delete gdb/testsuite/gdb.gdbtk
  http://thread.gmane.org/gmane.comp.gdb.patches/61489

and we should cleanup configure.ac accordingly.

gdb/testsuite:

2014-12-01  Yao Qi  <yao@codesourcery.com>

	* configure.ac: Remove AC_ARG_ENABLE for gdbtk.  Don't invoke
	AC_CONFIG_SUBDIRS(gdb.gdbtk).
	* configure: Re-generated.