(partially) Fix Bug#55227 Fix compiler warnings in innodb with gcc 4.6

Fix compiler warning:
trx/trx0undo.c: In function 'trx_undo_truncate_end':
trx/trx0undo.c:1069:14: error: variable 'rseg' set but not used [-Werror=unused-but-set-variable]

storage/innodb_plugin/trx/trx0undo.c

@@ -1066,14 +1066,11 @@ trx_undo_truncate_end(
 	ulint		last_page_no;
 	trx_undo_rec_t* rec;
 	trx_undo_rec_t* trunc_here;
-	trx_rseg_t*	rseg;
 	mtr_t		mtr;
 
 	ut_ad(mutex_own(&(trx->undo_mutex)));
 	ut_ad(mutex_own(&(trx->rseg->mutex)));
 
-	rseg = trx->rseg;
-
 	for (;;) {
 		mtr_start(&mtr);