Log Message:
mysql80: Update to 8.0.31.

Includes a number of pkgsrc cleanups, removing nonsense that has been
cargo-culted since the MySQL 3.23.x days(!).  Stop shipping mysql-test, it's
absolutely huge and unnecessary.  Add XXX to undocumented patches that we
should probably just drop, and make others easier to maintain.

Upstream changes are far too numerous to include here, so here are the links to
the corresponding release notes for each since 8.0.24:

  https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-31.html
  https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-30.html
  https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-29.html
  https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-28.html
  https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-27.html
  https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-26.html
  https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-25.html

Tested on SmartOS, I'll monitor bulk build reports for any fallout on other
platforms (it takes ages to build).