Remove thin gem from gitlab-org/gitlab repo

Epic: &8183 (closed)

TODO

• gitlab-development-kit#1511 (closed)
• gitlab-development-kit#1512 (closed)

Follow-ups

Following tasks will be managed in &8183 (closed):

• gitlab-development-kit!2723 (merged)
• gitlab-development-kit#1562 (closed)

Resources

• thin was introduced to GitLab only for development purpose in GitLab 4.1 by @dz probably to improve gitolite integration, which was removed from GitLab. 54a24608
• Discourse got rid of thin in 2017: https://github.com/discourse/discourse/commit/1a45fe94a284d7809abc9a64f371e58b80ec50f8
• Puma is the default web server, which can be used for development purpose.