patch 8.1.0785: depending on the configuration some functions are unused

Problem:    Depending on the configuration some functions are unused.
Solution:   Add more #ifdefs, remove unused functions. (Dominique Pelle,
            closes #3822)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 8a9b2f4..4722edd 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -12314,6 +12314,7 @@
 }
 #endif
 
+#if defined(FEAT_CMDL_COMPL) || defined(PROTO)
     char_u *
 get_mapclear_arg(expand_T *xp UNUSED, int idx)
 {
@@ -12321,6 +12322,7 @@
 	return (char_u *)"<buffer>";
     return NULL;
 }
+#endif
 
 static int filetype_detect = FALSE;
 static int filetype_plugin = FALSE;