[llvm][utils] Remove git-svn folder + scripts

Summary:
These tools are no longer useful since we've migrated off of SVN, so
this patch deletes them.

RFC Link: http://lists.llvm.org/pipermail/llvm-dev/2020-May/141386.html

Unless there is opposition in the RFC thread, I'll submit the patch on
May 10, 2020.

I searched through the repo to confirm there were no mentions of the scripts
in other scripts or documentation.

Reviewed By: echristo, tstellar, MaskRay

Differential Revision: https://reviews.llvm.org/D79348