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/buffer.c b/src/buffer.c
index 8691500..5a30aff 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -925,11 +925,9 @@
 	CHANGEDTICK(buf) = tick;
     }
 #endif
-#ifdef FEAT_USR_CMDS
-    uc_clear(&buf->b_ucmds);		/* clear local user commands */
-#endif
+    uc_clear(&buf->b_ucmds);		// clear local user commands
 #ifdef FEAT_SIGNS
-    buf_delete_signs(buf, (char_u *)"*");	// delete any signs */
+    buf_delete_signs(buf, (char_u *)"*");	// delete any signs
 #endif
 #ifdef FEAT_NETBEANS_INTG
     netbeans_file_killed(buf);