mined.h: remove irrlevant comment.

It used to refer to several NIL_* definitions. Those were removed
in commit 6e25ad8b0a leaving the
comment with nothing to comment on.

Change-Id: I52221ae5d2c5216e82391b480cf2038ad723b8f0

minix/commands/mined/mined.h

@@ -123,8 +123,6 @@ struct regex {
 
 typedef struct regex REGEX;
 
-/* NULL definitions */
-
 /*
  * Forward declarations
  */