patch 8.1.1210: support for user commands is spread out

Problem:    Support for user commands is spread out. No good reason to make
            user commands optional.
Solution:   Move user command support to usercmd.c.  Always enable the
            user_commands feature.
diff --git a/src/eval.c b/src/eval.c
index bf82d02..a1ad0e6 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1120,10 +1120,10 @@
     return retval;
 }
 
-#if (defined(FEAT_USR_CMDS) && defined(FEAT_CMDL_COMPL)) \
+#if defined(FEAT_CMDL_COMPL) \
 	|| defined(FEAT_COMPL_FUNC) || defined(PROTO)
 
-# if (defined(FEAT_USR_CMDS) && defined(FEAT_CMDL_COMPL)) || defined(PROTO)
+# if defined(FEAT_CMDL_COMPL) || defined(PROTO)
 /*
  * Call Vim script function "func" and return the result as a string.
  * Returns NULL when calling the function fails.