Remove glibc debug environment variables

These environment variables do not work anymore. See:

https://gitlab.com/nbdkit/nbdkit/-/commit/362e0fdcae37db876e13b944102a5c152e6bc563

so just remove them. It's broken after my 43.alpha release because I removed cheese_minor_version, because it relies on converting "alpha" to an int. I didn't notice because tests are disabled by default for some reason. CI is green, so presumably they're not running there either. Oh well.