Remove block-based table assertion for non-empty filter block

Summary:
7a6353bd1c prevents empty filter blocks from being written for SST files containing range deletions only. However the assertion this PR removes is still a problem as we could be reading from a DB generated by a RocksDB build without the 7a6353bd1c patch. So remove the assertion. We already don't do this check when `cache_index_and_filter_blocks=false`, so it should be safe.
Closes https://github.com/facebook/rocksdb/pull/3773

Differential Revision: D7769964

Pulled By: ajkr

fbshipit-source-id: 7285762446f2cd2ccf16efd7a988a106fbb0d8d3

table/block_based_table_reader.cc

@@ -1332,7 +1332,6 @@ BlockBasedTable::CachableEntry<FilterBlockReader> BlockBasedTable::GetFilter(
     filter =
         ReadFilter(prefetch_buffer, filter_blk_handle, is_a_filter_partition);
     if (filter != nullptr) {
-      assert(filter->size() > 0);
       Status s = block_cache->Insert(
           key, filter, filter->size(), &DeleteCachedFilterEntry, &cache_handle,
           rep_->table_options.cache_index_and_filter_blocks_with_high_priority