Remove unused Block member from MutateTask

v0: Fix excessive memory usage in MutateTask
    This is relevant only for 01200_mutations_memory_consumption test, since
    this is just a block.
v2: just remove unused Block member, since other part had been fixed in #29768

src/Storages/MergeTree/MutateTask.cpp

@@ -766,7 +766,6 @@ private:
     State state{State::NEED_PREPARE};
     MutationContextPtr ctx;
 
-    Block block;
     size_t block_num = 0;
 
     using ProjectionNameToItsBlocks = std::map<String, MergeTreeData::MutableDataPartsVector>;