delete unnessary pointer cast in beginInternalTransaction() function
delete unnessary pointer cast in beginInternalTransaction() function

Summary: use of dynamic_cast<TransactionImpl*> is unnecessary and also introduce difficulty for fbrocksdb support of TransactionDB

Test Plan: ./transaction_test

Reviewers: sdong, IslamAbdelRahman, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D60501

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