Remove set-but-unused variable.

Clean-up for commit c24dcd0c.

Reported-by: Andrew Dunstan
Discussion: https://postgr.es/m/2d52ff4a-5440-f6f1-7806-423b0e6370cb%402ndQuadrant.com

src/backend/storage/file/fd.c

@@ -2004,15 +2004,11 @@ FileSync(File file, uint32 wait_event_info)
 off_t
 FileSize(File file)
 {
-	Vfd		   *vfdP;
-
 	Assert(FileIsValid(file));
 
 	DO_DB(elog(LOG, "FileSize %d (%s)",
 			   file, VfdCache[file].fileName));
 
-	vfdP = &VfdCache[file];
-
 	if (FileIsNotOpen(file))
 	{
 		if (FileAccess(file) < 0)