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/edit.c b/src/edit.c
index 65c53f5..d960de3 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -516,7 +516,7 @@
      */
     if (curbuf->b_p_iminsert == B_IMODE_LMAP)
 	State |= LANGMAP;
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
     im_set_active(curbuf->b_p_iminsert == B_IMODE_IM);
 #endif
 
@@ -8372,7 +8372,7 @@
     ++no_u_sync;
     if (regname == '=')
     {
-# ifdef USE_IM_CONTROL
+# ifdef FEAT_MBYTE
 	int	im_on = im_get_status();
 # endif
 	/* Sync undo when evaluating the expression calls setline() or
@@ -8380,7 +8380,7 @@
 	u_sync_once = 2;
 
 	regname = get_expr_register();
-# ifdef USE_IM_CONTROL
+# ifdef FEAT_MBYTE
 	/* Restore the Input Method. */
 	if (im_on)
 	    im_set_active(TRUE);
@@ -8509,12 +8509,12 @@
 	{
 	    curbuf->b_p_iminsert = B_IMODE_LMAP;
 	    State |= LANGMAP;
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
 	    im_set_active(FALSE);
 #endif
 	}
     }
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
     else
     {
 	/* There are no ":lmap" mappings, toggle IM */
@@ -8661,7 +8661,7 @@
 	}
     }
 
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
     /* Disable IM to allow typing English directly for Normal mode commands.
      * When ":lmap" is enabled don't change 'iminsert' (IM can be enabled as
      * well). */