[kfinddialog] Remove usage of kregexpeditor plugin system

Summary:
The interface is only provided by one unreleased application, so people are used to use the fallback implementation.
This allows to drop the dependency on KService

Test Plan: Test KFindDialog with KAlarm

Reviewers: Frameworks, cullmann

Reviewed By: cullmann

Subscribers: cullmann, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12159

Differential Revision: https://phabricator.kde.org/D25538