implicit fallthough if there is fallthoughContext

Summary: fallthrough="true" is redundant with fallthroughContext="xxx" and there is no reason to use fallthrough="false"

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, dhaumann, cullmann

Subscribers: nibags, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23034