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/feature.h b/src/feature.h
index b900569..95701a7 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -379,10 +379,8 @@
 
 /*
  * +user_commands	Allow the user to define his own commands.
+ *			Now always enabled.
  */
-#ifdef FEAT_NORMAL
-# define FEAT_USR_CMDS
-#endif
 
 /*
  * +printer		":hardcopy" command