kcoreaddons_add_plugin: remove effectless OBJECT_DEPENDS on json file

Summary:
The JSON file argument passed to K_PLUGIN_FACTORY_WITH_JSON ends up
being used with the macro Q_PLUGIN_METADATA, which is a no-code macro
at the C++ level and only used to note information used by moc for the
generated moc file.

So when the content of the JSON file has changed, this will not change
anything in the preprocessed source file itself. It will only change the
content of the moc file generated based on it, which already is noted
as dependency to the object file due to being a file included by the
source.

This code possibly was added hoping to solve the issue of cmake's automoc
not properly dealing with Q_OBJECT macros hidden away in other C++
preprocessor macros. But it misses to catch the issue that it is the
generated moc file which needs to be regenerated on a change of the JSON
file, not just the object file.
Regenerating the moc file is something only automoc decides about when
being run and which seems not influenced by CMakeLists.txt rules.

Cmp. https://gitlab.kitware.com/cmake/cmake/issues/17750

Test Plan:
Projects like plasma-desktop still build as before.
Editing desktop files/JSON files still results in same (broken) effective rebuild
behaviour: the moc file is not regenerated, so the new JSON content
not picked up. Other than before the unneeded-because-all-sources-the-same
rebuild of the object file is skipped though.

Reviewers: Frameworks, apol

Reviewed By: apol

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10665