Remove the orphan assert on !need_log_sync

Summary:
We initially had disabled support for write_options.sync when concurrent_prepare_ is set. We later added this support but the statement that asserts this combination is not used was left there. This patch cleans it up.
Closes https://github.com/facebook/rocksdb/pull/2642

Differential Revision: D5496101

Pulled By: maysamyabandeh

fbshipit-source-id: becbc503446f2a51bee24cc861958c090c724ec2

db/db_impl_write.cc

@@ -230,7 +230,6 @@ Status DBImpl::WriteImpl(const WriteOptions& write_options,
                             need_log_dir_sync, last_sequence + 1);
       }
     } else {
-      assert(!need_log_sync && !need_log_dir_sync);
       if (status.ok() && !write_options.disableWAL) {
         PERF_TIMER_GUARD(write_wal_time);
         // LastToBeWrittenSequence is increased inside WriteToWAL under