Drop a sqlite_version check because we only support SQLite 3.6.16 or newer.

Drop some comments that document the implementation rather than the interface.
Remove the unused, undocumented SQLite3Adapter#supports_count_distinct?
    
This has returned true since 3cc9b5f1, and is not used internally.
Merge pull request #11921 from Empact/sqlite-3-cleanup

Remove the unused, undocumented SQLite3Adapter#supports_count_distinct?