Remove unused persistentsetmap.h

Summary:
Added 2008 in e13432be5a021c8e2116e0d2a4114e53f612a4dc but no code found
committed which picks up that header and its classes

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9210