Remove some unneeded code (#8736)

Remove some unneeded code (#8736)

Summary:
* FullKey and ParseFullKey appear to serve no purpose in the public API
(or anything else) so removed. Only use in one test updated.
* NumberToString serves no purpose vs. ToString so removed, numerous
calls updated
* Remove unnecessary forward declarations in metadata.h by re-arranging
class definitions.
* Remove some unneeded semicolons

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8736

Test Plan: existing tests

Reviewed By: mrambacher

Differential Revision: D30700039

Pulled By: pdillinger

fbshipit-source-id: 1e436a576f511a6ed8b4d97af7cc8216bc729af2