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.
Actually this is wrong. We should first check the credentials on disk (`~/.subversion`) before using the credentials plugin.

We should be calling `super.getFirstAuthentication(...)` then on failure, use credentials from the credentials plugin.