Fix backupable db test

Summary:
#1733 started using SizeFileBytes(), so our dummy log file implementation should stop asserting that this function isn't called.
Closes https://github.com/facebook/rocksdb/pull/1740

Differential Revision: D4376055

Pulled By: ajkr

fbshipit-source-id: 2854d89

utilities/backupable/backupable_db_test.cc

@@ -112,8 +112,6 @@ class DummyDB : public StackableDB {
     }
 
     virtual uint64_t SizeFileBytes() const override {
-      // backupabledb should not need this method
-      EXPECT_TRUE(false);
       return 0;
     }