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.
Is this change actually specific to this PR, or could it be done separately in trunk?

(BTW at some point it would be good to replace usages of `HudsonTestCase` with `JenkinsRule`; then you could use `org.junit.Assume` to have the test be considered skipped rather than failed if there is an infrastructure problem. Of course it would be better still to select an arbitrary open port.)