crimson/os/seastore/btree: drop unused lines

Signed-off-by: Xuehan Xu <[email protected]>

Author: xxhdx1985126

src/crimson/os/seastore/btree/fixed_kv_btree.h

@@ -1068,7 +1068,6 @@ class FixedKVBtree {
       });
     };
 
-    CachedExtentRef n_fixed_kv_extent;
     if (e->get_type() == internal_node_t::TYPE) {
       auto lint = e->cast<internal_node_t>();
       return do_rewrite(*lint);