Subvertpy hasn't had a release in two years, hasn't kept up with libsvn,

and has ended up more problematic for us than helpful. Since its

introduction, we've basically solved the PySVN installation story. With

the release of Review Board 5, we documented that Subvertpy was on its

way out.
This change completes that deprecation, removing all Subvertpy support

from the codebase and providing a notice in the documentation.
The Subversion SCMTool now relies on PySVN fully. This will end up

helping with some cleanup work that's in progress for SSL cert

management.
The backend support remains. This may prove useful if we end up crafting

a ctypes-based backend down the road.

Unit tests pass.
Checked the codebase for any more signs of Subvertpy, and verified they

were all removed.