refactored theme delete code to doctrine2.

This PR converts an API I missed in #1320.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| Refs tickets  | #1320
| License       | MIT
| Doc PR        | zikula/zikula-docs#123