Remove modified svnkit library from subversion plugin.

Currently the subversion plugin is using a modified version of the svnkit library. This PR switches to the default svnkit in order to make svnkit upgrading easier.
This deserves some explanation. Are these files indeed copied from SVNKit sources? If so, why are we not simply using the originals? If not, why this license notice? If these files are merely _inspired_ by classes in SVNKit, but in fact modified by Jenkins contributors, then the details should be given. If the files are only subclassing SVNKit classes to add fiurther behaviors, but not reusing source code from SVNKit, then they would be under the same MIT license as the rest of the plugin.