[RocksDB] [Performance Branch] Minor fix, Remove string resize from WriteBatch::Clear

Summary: tmp_batch_ will get re-allocated for every merged write batch because of the existing resize in WriteBatch::Clear. Note that in DBImpl::BuildBatchGroup, we have a hard coded upper limit of batch size 1<<20 = 1MB already.

Test Plan: make check

Reviewers: dhruba, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14787