Remove an assert that causes TSAN failure.

Summary:
ColumnFamilyData::ConstructNewMemtable is called out of DB mutex, and it asserts current_ is not empty, but current_ should only be accessed inside DB mutex. Remove this assert to make TSAN happy.
Closes https://github.com/facebook/rocksdb/pull/2235

Differential Revision: D4978531

Pulled By: siying

fbshipit-source-id: 423685a7dae88ed3faaa9e1b9ccb3427ac704a4b

db/column_family.cc

@@ -779,7 +779,6 @@ uint64_t ColumnFamilyData::GetTotalSstFilesSize() const {
 
 MemTable* ColumnFamilyData::ConstructNewMemtable(
     const MutableCFOptions& mutable_cf_options, SequenceNumber earliest_seq) {
-  assert(current_ != nullptr);
   return new MemTable(internal_comparator_, ioptions_, mutable_cf_options,
                       write_buffer_manager_, earliest_seq);
 }