Issue/248 - CPT/Taxos: unwind deep integration.

[JJJ]

This was only partially working, and after further review was a bit too aggressive with everything that it attempted to rearrange.

This commit introduces sugar_calendar_admin_is_assets_page() and sugar_calendar_is_supported_type() helper functions, and uses them where helpful.

This PR removes the way that supported post types (other than "sc_events") would be redirected to the main admin area calendar. It will soon explore a better way.

See #248