Clean up the communication of where development happens

Problem/Motivation

Currently, the project page → suggests that development happens on GitHub. But the link provided here leads to an archived repository. The ./DEVELOPING.md file also directs the reader to two different places.

However, with the recent introduction of stable development services of GitLab provided by the Drupal Association, moving Drupal development away from D․org slowly became less popular and we see many projects returning back to the mothership.

Proposed resolution

Therefore I would suggest the following changes to module maintainers:

1. Delete any occurrences of the once-living GitHub repository from all written documentation (eg. the project page) where it is mentioned in a general context. Leave traces only when a specific issue is being referenced on GH.
2. Delete the aforementioned ./DEVELOPING.md file from the codebase. It has served as a signpost directing users to different locations but once these locations are united there is no need for signposting either.
3. If the current ./.travis.yml file was used to configure CI jobs on GitHub, then consider converting this file into a ./.gitlab-ci.yml into instead.