Clean up environment variables defined in desimodules

[weaverba137]

As of desihub/desimodules#52, some environment variables will be defined in desimodules.

Please wait until the PR mentioned above is merged and deployed before merging.

[coveralls]

coverage: 53.05%. remained the same
when pulling cc56675 on env-var-clean-up
into 957645b on main.

[sbailey]

FTR: I merged, updated changes.rst, reinstalled desitarget/main at NERSC, and verified that these environment variables are still set by "source desi_environment.sh main", but unlike before they are no longer unset by "module unload desitarget" (which is a good thing, because they are also needed by desispec).