kr_rplan_next: remove prototype

It had no implementation for years - since 456e5446.

(cherry picked from commit 59126a77)
The commit was apparently "reverted" unintentionally when resolving
conflicts in a5b14c25.

lib/rplan.h

@@ -207,9 +207,6 @@ struct kr_query *kr_rplan_resolved(struct kr_rplan *rplan);
 KR_EXPORT KR_PURE
 struct kr_query *kr_rplan_last(struct kr_rplan *rplan);
 
-/** Return query predecessor. */
-KR_EXPORT KR_PURE
-struct kr_query *kr_rplan_next(struct kr_query *qry);
 
 /**
  * Check if a given query already resolved.