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.
Was this really written by you? If not, retain the original copyright and license notice from wherever you got this from. And put it into a distinct package with a `package-info.java` explaining the origin and anything else of interest to future maintainers.

(If this is from the patched version of SVNKit, please be sure there is an upstream issue filed to incorporate the changes we want into some upcoming official release, and include a notice here (say, in `package-info`) of what that issue is; and if and when closed with a particular target milestone, add a `TODO` comment here requesting that the custom code be replaced with the official version when upgrading to that version.)