Summary:
Reorganize the code so that no function call into ReadRangeDelAggregator is needed if there is no tomb range stone.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5202

Differential Revision: D14968155

Pulled By: siying

fbshipit-source-id: 0bd61911293c7a27b4e1b8d57c66d0c4ad6a6a5f