Remove "show" command completers, "set" command completers for string commands

Remove "show" command completers, "set" command completers for string commands

The default command completer is symbol_completer, but it makes no
sense for a "show" command to complete on symbols, or anything else,
really.

I wonder whether we should instead make the default be no completer.
That seems like a much larger/complicated audit/change, so I'd like to
move forward with this version, as it'll be covered by tests.  I
noticed this because a following patch will add a new
gdb.base/settings.exp testcase that exercises all sorts of details of
settings commands, including completing the show commands, using new
representative "maint test-settings <type or settings command>"
commands.

Also remove the completer for var_string and var_string_noescape
commands.  No point in completing symbols when GDB is expecting a
string.

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* cli/cli-decode.c (add_setshow_cmd_full): Remove "show"
	completer.
	(add_setshow_string_cmd, add_setshow_string_noescape_cmd): Remove
	"set" completers.