Sequel to #8836

After finding a Makefile error when updating xz to 5.8.0 on manylinux2014, I asked in the xz repository about this - tukaani-project/xz#170 (comment)

The initial response suggested changing our build to handle the fact that xz 5.8.0 no longer worked out of the box for our environment. I didn't immediately consider manylinux2014 a priority, and thought we could live with the older version until Amazon Linux 2 goes EOL in a year and a bit (if Amazon doesn't extend support again) and we drop manylinux2014 support.

However, it turns out that xz would like us to use the latest version. So this PR uses one of their suggestions, removing po4a.

https://tukaani.org/xz/#_optional_dependencies

Optional dependencies
po4a is needed for translated documentation (man pages).

Autotools: To build without po4a, pass --no-po4a as the argument to autogen.sh.

They have also said that this is a temporary situation, and that xz 5.8.1 will restore support.