Remove unused external declaration

The function DefineForeignRelation referenced in utility.c was removed
in 601c58c3.
Signed-off-by: NTaylor Vesely <tvesely@pivotal.io>

src/include/commands/tablecmds.h

@@ -47,8 +47,6 @@ extern Oid	DefineRelation(CreateStmt *stmt, char relkind, char relstorage, bool
 
 extern void	DefineExternalRelation(CreateExternalStmt *stmt);
 
-extern void DefineForeignRelation(CreateForeignStmt *createForeignStmt);
-
 extern void	DefinePartitionedRelation(CreateStmt *stmt, Oid reloid);
 
 extern void EvaluateDeferredStatements(List *deferredStmts);