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

Fix compiler warning:
trx/trx0undo.c: In function 'trx_undo_set_state_at_prepare':
trx/trx0undo.c:1798:16: error: variable 'page_hdr' set but not used [-Werror=unused-but-set-variable]

storage/innobase/trx/trx0undo.c

@@ -1795,7 +1795,6 @@ trx_undo_set_state_at_prepare(
 	mtr_t*		mtr)	/* in: mtr */
 {
 	trx_usegf_t*	seg_hdr;
-	trx_upagef_t*	page_hdr;
 	trx_ulogf_t*	undo_header;
 	page_t*		undo_page;
 	ulint		offset;
@@ -1812,7 +1811,6 @@ trx_undo_set_state_at_prepare(
 	undo_page = trx_undo_page_get(undo->space, undo->hdr_page_no, mtr);
 
 	seg_hdr = undo_page + TRX_UNDO_SEG_HDR;
-	page_hdr = undo_page + TRX_UNDO_PAGE_HDR;
 
 	/*------------------------------*/
 	undo->state = TRX_UNDO_PREPARED;