Remove the unused, undocumented SQLite3Adapter#supports_count_distinct?

These have returned true since 3cc9b5f1, are only used internally and in tests.

Also drop a sqlite_version check because we only support 3.6.16+ now.

A few things this prompts:
* `#default_primary_key_type` is defunct but documented
* `#supports_primary_key?` is not used internally, but is documented in `AbstractAdapter` and `AbstractMysqlAdapter`
* `#sqlite_version` is also unused internally, but that seems like it may be useful.

Should we deprecate and remove these?