patch 8.0.1338: USE_IM_CONTROL is confusing and incomplete

Problem:    USE_IM_CONTROL is confusing and incomplete.
Solution:   Just use FEAT_MBYTE.  Call 'imactivatefunc' also without GUI.
diff --git a/src/gui.c b/src/gui.c
index 8118ca1..4f03ed8 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -1078,7 +1078,7 @@
 	gui_undraw_cursor();
 	if (gui.row < 0)
 	    return;
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
 	if (gui.row != gui.cursor_row || gui.col != gui.cursor_col)
 	    im_set_position(gui.row, gui.col);
 #endif
@@ -1136,7 +1136,7 @@
 	if (id > 0)
 	{
 	    cattr = syn_id2colors(id, &cfg, &cbg);
-#if defined(USE_IM_CONTROL) || defined(FEAT_HANGULIN)
+#if defined(FEAT_MBYTE) || defined(FEAT_HANGULIN)
 	    {
 		static int iid;
 		guicolor_T fg, bg;