Remove unused TransactionCallback
Remove unused TransactionCallback

Summary:
TransactionCallback was never used. Remove it to avoid confusion.
Closes https://github.com/facebook/rocksdb/pull/2853

Differential Revision: D5787219

Pulled By: maysamyabandeh

fbshipit-source-id: e2b6a89537e3770a269ad38be71c4b0b160a88ac

bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb