Remove SVN.md

[vincentdavis]

This page is no longer relevant and the GitMigration.md covers the migration, not sure that is still necessary.
https://github.com/biopython/biopython.github.io/blob/master/wiki/SVN.md

https://github.com/biopython/biopython.github.io/blob/master/wiki/GitMigration.md

[peterjc]

Its already a stub describing why we never used SVN, could make it into a redirect to the source code page?

[peterjc]

I'm going to close this as a duplicate of #28 since we'll fix them both the same way.