lister.github.utils: Drop no longer used module

This got detected when working on the deployment of the new loader-git.

loader-{git,save-code-now,add-forge-now,...} images -depend-> swh-loader-metadata -depend-> swh-lister (with a new deps rpy2 which breaks build)

loader-metadata solely used to* [1] depend on this module.

Refs. swh/infra/sysadm-environment#5017 (closed)

[1] swh-loader-metadata!17 (merged)