[llvm][utils] Remove git-svn folder + scripts
These tools are no longer useful since we've migrated off of SVN, so
this patch deletes them.
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