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.
Thinking about this a little more, these changes shouldn't be persisted upstream because `DefaultSVNAuthenticationManager` should only consist of authentication either from cache or `~/.subversion` directory and ignore anything else.

I agree, package will be also changed.