Remove --disable-gdbcli

Remove --disable-gdbcli

I think it doesn't really make sense to allow building gdb without the
CLI.  Perhaps at one time this was a goal, but libgdb is long gone and
the CLI is intrinsic to gdb.

So, this patch removes the implementation of this configure option.
It is still recognized (this is autoconf's default), but does nothing.

This simplifies configure.ac and Makefile.in a bit.

Tested by rebuilding.

gdb/ChangeLog
2018-07-17  Tom Tromey  <tom@tromey.com>

	* configure.ac: Remove --disable-gdbcli.
	* configure: Rebuild.
	* Makefile.in (SUBDIR_CLI_DEPS, SUBDIR_CLI_LDFLAGS)
	(SUBDIR_CLI_CFLAGS): Remove.
	(SFILES): Use SUBDIR_CLI_SRCS.
	(COMMON_OBS): Use SUBDIR_CLI_OBS.