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

Fix compiler warning:
btr/btr0btr.c: In function 'btr_compress':
btr/btr0btr.c:2564:9: error: variable 'level' set but not used [-Werror=unused-but-set-variable]

storage/innodb_plugin/btr/btr0btr.c

@@ -2561,7 +2561,6 @@ btr_compress(
 	ulint		n_recs;
 	ulint		max_ins_size;
 	ulint		max_ins_size_reorg;
-	ulint		level;
 
 	block = btr_cur_get_block(cursor);
 	page = btr_cur_get_page(cursor);
@@ -2571,7 +2570,6 @@ btr_compress(
 	ut_ad(mtr_memo_contains(mtr, dict_index_get_lock(index),
 				MTR_MEMO_X_LOCK));
 	ut_ad(mtr_memo_contains(mtr, block, MTR_MEMO_PAGE_X_FIX));
-	level = btr_page_get_level(page, mtr);
 	space = dict_index_get_space(index);
 	zip_size = dict_table_zip_size(index->table);