Fix a bug in KCM cursor theme, after applying change, cursor preview don't work

Summary:
This fix work only after applying other patch (but fix an issue that exist on master)  (24519)
Bug: When you go to the kcm cursor,  if you go hover a theme the "real" cursor change to show a preview. After applying a new theme the preview don't work anymore before you change the cursor size.
In order to work we need a call to setTheme that is done by setCursorSize.
Currently the  if (m_currentSize == size) prevent the call to setTheme, so I removed it and removed the unused signal

Reviewers: #plasma, mart, ervin, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24520