fix valgrind leak report in unit test

Summary:
I cannot locally reproduce the valgrind leak report but based on my code inspection not deleting txn1 might be the reason.
```
==197848== 2,990 (544 direct, 2,446 indirect) bytes in 1 blocks are definitely lost in loss record 15 of 16
==197848==    at 0x4C2D06F: operator new(unsigned long) (in /usr/local/fbcode/gcc-5-glibc-2.23/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==197848==    by 0x7D5B31: rocksdb::WritePreparedTxnDB::BeginTransaction(rocksdb::WriteOptions const&, rocksdb::TransactionOptions const&, rocksdb::Transaction*) (pessimistic_transaction_db.cc:173)
==197848==    by 0x7D80C1: rocksdb::PessimisticTransactionDB::Initialize(std::vector<unsigned long, std::allocator<unsigned long> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> > const&) (pessimistic_transaction_db.cc:115)
==197848==    by 0x7DC42F: rocksdb::WritePreparedTxnDB::Initialize(std::vector<unsigned long, std::allocator<unsigned long> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> > const&) (pessimistic_transaction_db.cc:151)
==197848==    by 0x7D8CA0: rocksdb::TransactionDB::WrapDB(rocksdb::DB*, rocksdb::TransactionDBOptions const&, std::vector<unsigned long, std::allocator<unsigned long> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> > const&, rocksdb::TransactionDB**) (pessimistic_transaction_db.cc:275)
==197848==    by 0x7D9F26: rocksdb::TransactionDB::Open(rocksdb::DBOptions const&, rocksdb::TransactionDBOptions const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> >*, rocksdb::TransactionDB**) (pessimistic_transaction_db.cc:227)
==197848==    by 0x7DB349: rocksdb::TransactionDB::Open(rocksdb::Options const&, rocksdb::TransactionDBOptions const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::TransactionDB**) (pessimistic_transaction_db.cc:198)
==197848==    by 0x52ABD2: rocksdb::TransactionTest::ReOpenNoDelete() (transaction_test.h:87)
==197848==    by 0x51F7B8: rocksdb::WritePreparedTransactionTest_BasicRecoveryTest_Test::TestBody() (write_prepared_transaction_test.cc:843)
==197848==    by 0x857557: HandleSehExceptionsInMethodIfSupported<testing::Test, void> (gtest-all.cc:3824)
==197848==    by 0x857557: void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (gtest-all.cc:3860)
==197848==    by 0x84E7EB: testing::Test::Run() [clone .part.485] (gtest-all.cc:3897)
==197848==    by 0x84E9BC: Run (gtest-all.cc:3888)
==197848==    by 0x84E9BC: testing::TestInfo::Run() [clone .part.486] (gtest-all.cc:4072)
```
Closes https://github.com/facebook/rocksdb/pull/2963

Differential Revision: D5968856

Pulled By: maysamyabandeh

fbshipit-source-id: 2ac512bbcad37dc8eeeffe4f363978913354180c

utilities/transactions/write_prepared_transaction_test.cc

@@ -869,6 +869,7 @@ TEST_P(WritePreparedTransactionTest, BasicRecoveryTest) {
   txn1 = db->GetTransactionByName("xid" + istr1);
   ASSERT_NE(txn1, nullptr);
   txn1->Commit();
+  delete txn1;
 
   index++;
   Transaction* txn2 = db->BeginTransaction(write_options, txn_options);