Remove unneccessary MutexLock from NonBatchedOpStressTest::TestGetEntity

Summary: This is most likely copypasta from `TestGet` from before facebook#11058 . There is no need to lock the mutex for the key for reads; in fact, doing so is detrimental to test coverage since it locks out concurrent writers.

Differential Revision: D57915207

db_stress_tool/no_batched_ops_stress.cc

@@ -901,17 +901,14 @@ class NonBatchedOpsStressTest : public StressTest {
     assert(shared);
 
     assert(!rand_column_families.empty());
-    assert(!rand_keys.empty());
-
-    std::unique_ptr<MutexLock> lock(new MutexLock(
-        shared->GetMutexForKey(rand_column_families[0], rand_keys[0])));
-
     assert(rand_column_families[0] >= 0);
     assert(rand_column_families[0] < static_cast<int>(column_families_.size()));
 
     ColumnFamilyHandle* const cfh = column_families_[rand_column_families[0]];
     assert(cfh);
 
+    assert(!rand_keys.empty());
+
     const std::string key = Key(rand_keys[0]);
 
     PinnableWideColumns columns_from_db;