patch 9.0.1403: unused variables and functions

Problem:    Unused variables and functions.
Solution:   Delete items and adjust #ifdefs. (Dominique Pellé, closes #12145)
diff --git a/src/autocmd.c b/src/autocmd.c
index a409379..fe2b6cb 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -2519,6 +2519,7 @@
     }
 }
 
+#if defined(FEAT_EVAL) || defined(PROTO)
 /*
  * Get the script context where autocommand "acp" is defined.
  */
@@ -2527,6 +2528,7 @@
 {
     return &acp->script_ctx;
 }
+#endif
 
 /*
  * Get next autocommand command.