Remove ExampleTheme plugin on update #16807

We already deactivate the plugin but it looks like we actually need to fully remove the plugin. It should only be removed if it was not installed through git as in git we still have it.

fix https://github.com/matomo-org/matomo/issues/16804

Review

• [ ] Functional review done
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done see checklist
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

Tests are passing