Remove `ps_tag` schema and its contents

[sumerman]

It appears to be a holdover from before we switched to tag_map + support functions (#189 and follow-ups).

• migration/idempotent/002-tag-operators.sql defines a bunch of functions. Probably can remove entirely.
• migration/incremental/005-tag-operators.sql creates types. Probably need to add corresponding drops in a later incremental migration. Unless they are still used by matcher operators for series.
• Snapshot tests may require adjustments.