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/ui.c b/src/ui.c
index 1b2b3c5..7d969fe 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -3307,7 +3307,7 @@
 }
 #endif
 
-#if defined(USE_IM_CONTROL) || defined(PROTO)
+#if defined(FEAT_MBYTE) || defined(PROTO)
 /*
  * Save current Input Method status to specified place.
  */