Remove assert in vector rep

Summary: This assert makes Insert O(n^2) instead of O(n) in debug mode. Memtable insert is in the critical path. No need to assert uniqunnes of the key here, since we're adding a sequence number to it anyway.

Test Plan: none

Reviewers: sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22443

util/vectorrep.cc

@@ -106,7 +106,6 @@ class VectorRep : public MemTableRep {
 
 void VectorRep::Insert(KeyHandle handle) {
   auto* key = static_cast<char*>(handle);
-  assert(!Contains(key));
   WriteLock l(&rwlock_);
   assert(!immutable_);
   bucket_->push_back(key);