gdb: remove context parameter from add_setshow_enum_cmd

gdb: remove context parameter from add_setshow_enum_cmd

I propose removing the context parameter from add_setshow_enum_cmd.  It
was useful before add_setshow_enum_cmd returned both created commands,
as the caller couldn't easily set the context itself.  But now, I think
it's fine to just let the caller do it.

gdb/ChangeLog:

	* command.h (add_setshow_enum_cmd): Remove context parameter.
	* cli/cli-decode.c (add_setshow_enum_cmd): Likewise, and don't
	set context.
	* cli/cli-style.c (cli_style_option::add_setshow_commands): Set
	context here.

Change-Id: I377c4e6820ec9d5069492ed28f4cba342ce1336e