Fix SVN history fetching and clean up plugin

Summary:

• fetching the history of an svn project only worked sometimes
• trying to show a diff between versions lead to a crash
• displaying a diff from the annotation border lead to a crash

• remove UI files which are not used
• remove unused svncatjob and svncheckoutmetadatawidget

Test Plan:

• display history/log from vcs context menu entry
• display diff from history list
• display diff from annotation border
• fetch svn project using the assistant
• commit changes of an svn project

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9956