Remove thin gem

thin was introduced to GitLab only for development purpose in 54a24608
in GitLab 4.1 probably to improve gitolite integration, which was
removed from GitLab.

Puma is now the default web server, which can be used normally for
development purpose.

GitLab 4.1: https://about.gitlab.com/releases/2013/01/22/gitlab-4-1-released/

https://gitlab.com/gitlab-org/gitlab/-/commit/54a24608a29d113f0c607027cddd8b6947c6a55c



Changelog: other

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>