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/globals.h b/src/globals.h
index b479579..9f1864d 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1022,7 +1022,7 @@
 
 EXTERN int	KeyTyped;		/* TRUE if user typed current char */
 EXTERN int	KeyStuffed;		/* TRUE if current char from stuffbuf */
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
 EXTERN int	vgetc_im_active;	/* Input Method was active for last
 					   character obtained from vgetc() */
 #endif