Delete the temporary file used in buffered GiST build, after the build.

There were two bugs here: We forgot to call gistFreeBuildBuffers() function
at the end of build, and we passed interXact == true to BufFileCreateTemp,
so the file wasn't automatically cleaned up at end-of-transaction either.

src/backend/access/gist/gistbuild.c

@@ -214,6 +214,7 @@ gistbuild(PG_FUNCTION_ARGS)
 	{
 		elog(DEBUG1, "all tuples processed, emptying buffers");
 		gistEmptyAllBuffers(&buildstate);
+		gistFreeBuildBuffers(buildstate.gfbb);
 	}
 
 	/* okay, all heap tuples are indexed */