SvnLoaderFromRemoteDump: Drop dump as soon as it's no longer required
This will decrease the disk pressure currently existing when ingesting a svn repository
out of a remote dump. We first fetch a dump, then mount a svn repository out of it, and
at last we ingest the repository and have a growing copy on disk [1].
So we are up to 3 copies which takes lots of disk space. With the following, we take
down the 1 unnecessary copy as soon as possible.
[1] implementation detail for speed.
Related to T3719