removed obsolete Sf2 forms plugin

This removes the plugin as well as some unrequired form classes.
After this is merged there are still some classes in `lib/Zikula/Core/Form/` but these remaining ones look interesting for us.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #2107 
| Refs tickets  | -
| License       | MIT
| Doc PR        | -