cleanup transients in option THEATER_TRANSIENTS_OPTION

[zupdo]

Fix to: remove nonexisting transients from transient list (option THEATER_TRANSIENTS_OPTION).

Steps to reproduce:

• open Kalendar, some Productions and/or Events in Frontend
• check existing transients
  • wp option get theater_transient_keys
  • wp transient get 'one from list above'
• wait more than 10 minutes
• open wp-admin, open some production
• check transients again (see above)
  • current result: transients exist in option theater_transient_keys, transients are empty
  • expected: option theater_transient_keys should be empty

For long running sites this leads to a huge amount of entries in option theater_transient_keys.
Each of them is reset again and again on many actions in wp-admin.
This leads to really slow backend pages.

About the change:
wordpress delete_transient() returns false if transient does not exist (anymore).
This is not an error condition. So removing the transient key from theater_transient_keys should always happen.