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/getchar.c b/src/getchar.c
index aa65a8c..18dc1a6 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -2890,7 +2890,7 @@
 						     + typebuf.tb_len] != NUL)
 			typebuf.tb_noremap[typebuf.tb_off
 						 + typebuf.tb_len++] = RM_YES;
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
 		    /* Get IM status right after getting keys, not after the
 		     * timeout for a mapping (focus may be lost by then). */
 		    vgetc_im_active = im_get_status();