The function interp_ui_out simply calls the interp_ui_out method.
However, if it is passed a NULL interpreter, it first finds the
current interpreter.  I believe, though, that NULL is never passed
here, and I think it's simpler to just remove this function and
require callers to be more explicit.

ChangeLog
2018-05-25  Tom Tromey	<tom@tromey.com>

	* utils.c (fputs_maybe_filtered): Update.
	* linespec.c (decode_line_full): Update.
	* mi/mi-interp.c (mi_on_normal_stop_1, mi_tsv_modified)
	(mi_print_breakpoint_for_event, mi_solib_loaded)
	(mi_solib_unloaded, mi_command_param_changed, mi_memory_changed)
	(mi_user_selected_context_changed): Update.
	* mi/mi-main.c (mi_execute_command): Update.
	* cli/cli-script.c (execute_control_command): Update.
	* python/python.c (execute_gdb_command): Update.
	* solib.c (info_sharedlibrary_command): Update.
	* interps.c (interp_ui_out): Remove.
	* interps.h (interp_ui_out): Remove.