Remove thin

[mayorova]

What this PR does / why we need it:

I am not sure whether this is used anymore. I tracked down the history, and it seems that it was added for some "proxy tests": https://github.com/3scale/system/commit/154af3a533a2536199c6852b48fb8c1d29e3cb9c

And these tests were already removed in https://github.com/3scale/porta/pull/2549/files

Well, now it's causing an issue when trying to upgrade rails to version 7.1:

Bundler could not find compatible versions for gem "rack":
  In Gemfile:
    rails (~> 7.1.5) was resolved to 7.1.5, which depends on
      actionpack (= 7.1.5) was resolved to 7.1.5, which depends on
        rack-session (>= 1.0.1) was resolved to 2.1.1, which depends on
          rack (>= 3.0.0)

    thin was resolved to 1.8.2, which depends on
      rack (>= 1, < 3)

It looks like the support was added to master branch: macournoyer/thin#399 However, it has not been released yet, the issue is still open: macournoyer/thin#389

Which issue(s) this PR fixes

Verification steps

Special notes for your reviewer: