Summary:
On Travis, the old `alignment()` returned by `RandomAccessFileReaderTest` is inconsistent with the `GetRequiredBufferAlignment` returned in `RandomAccessFileReader`. This PR removes `alignment()` and consistently use `GetRequiredBufferAlignment` as page size.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7173

Test Plan:
make random_access_file_reader_test && ./random_access_file_reader_test
Watch Travis

Reviewed By: siying

Differential Revision: D22741606

Pulled By: cheng-chang

fbshipit-source-id: f28f29a7c993bbc3594ae70ecd186fa8bab9c4f2