Port kpColorCollection away from KIO::NetAccess

Summary:
While testing "save as" over an existing color file, I noticed
a double confirmation message box. This is because QFileDialog
already takes care of it ==> simplify the code.

Test Plan: Loading colors from submenu, saving as, saving as existing file, loading from file.

Reviewers: broulik, andreagenor, tcanabrava, vkrause

Reviewed By: vkrause

Maniphest Tasks: T11576

Differential Revision: https://phabricator.kde.org/D25480