Remove cursor theme option

Summary: Continuation of D22191 and is built on top of it. Instead of having a niche option for a cursor theme with all the underlying infrastructure having to support it, grab the cursor them from a KDE config file and write it to the SDDM config file.

Reviewers: ngraham, davidedmundson

Reviewed By: ngraham, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22363