Code review comment for ~racb/git-ubuntu:cryptography-pin

This MP is no longer necessary since we moved the snap to core20 to base it on Focal. python3-cryptography from Focal is now used, and we rely on Ubuntu's security support for that package. In the case of a future bump to a core22, that Ubuntu release's build of python3-cryptography will be used, which will either already be built with Rust, or use whatever other solution Debian and Ubuntu will have found, and security support will continue through the distribution for us.