Remove custom doxygen config files

Summary:
Are not integrated wih kapidox or ECMAddQch, and the other
KF5 modules do not have such files.

For custom generation of API dox instead the QCH generation
can be used, by the target KF5I18n_QCH:

make KF5I18n_QCH

And either the created QCH file be used
or the intermediate html files in
src/KF5I18n_ECMQchDoxygen/html/ below the build dir
be looked at with a normal web browser.

By using some official API dox generation method this will
also ensure the output uses official configurations and any
issues might not be missed.

Reviewers: Frameworks, ilic

Reviewed By: ilic

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6339