Remove misleading use of KDE_INSTALL_PYTHONBINDINGSDIR
Summary:
Removing the (typically empty) optional argument from the function
invocation avoids confusion and allows to remove the parameter.
See also D15558
Test Plan: make
Reviewers: Frameworks, apol
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D15848