Revert "Merge branch '6078-deprecate-unicorn' into 'master'"

What does this MR do?

This reverts !5180 (merged) after receiving the following error during the nightly upgrade on dev.gitlab.org (incident issue production#4409)

================================================================================
Recipe Compile Error in /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/default.rb
================================================================================

NoMethodError
-------------
undefined method `empty?' for false:FalseClass

Cookbook Trace:
---------------
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/deprecations.rb:222:in `deprecate_only_if_value'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/deprecations.rb:215:in `additional_deprecations'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/deprecations.rb:208:in `check_config'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/omnibus_helper.rb:204:in `check_deprecations'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/default.rb:28:in `from_file'

Relevant File Content:
----------------------
/opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/deprecations.rb:

215:          messages += deprecate_only_if_value(incoming_version, existing_config, type, ['gitlab', 'unicorn'], 'enable', true, '13.10', '14.0')
216:
217:          messages
218:        end
219:
220:        def deprecate_only_if_value(incoming_version, existing_config, type, config_keys, key, value, deprecated_version, removed_version)
221:          setting = existing_config.dig(*config_keys) || {}
222>>         return [] if setting[key].nil? || setting[key].empty?
223:
224:          return [] if type == :removal && setting[key] != value
225:
226:          config_keys.shift if ATTRIBUTE_BLOCKS.include?(config_keys[0])
227:          messages = []
228:
229:          if Gem::Version.new(incoming_version) >= Gem::Version.new(removed_version) && type == :removal
230:            messages << "* #{config_keys[0]}[#{key}] has been deprecated since #{deprecated_version} and was removed in #{removed_version}."
231:          elsif Gem::Version.new(incoming_version) >= Gem::Version.new(deprecated_version) && type == :deprecation

System Info:
------------
chef_version=15.14.0
platform=ubuntu
platform_version=16.04
ruby=ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux]
program_name=/opt/gitlab/embedded/bin/chef-client
executable=/opt/gitlab/embedded/bin/chef-client


Running handlers:
There was an error running gitlab-ctl reconfigure:

undefined method `empty?' for false:FalseClass

Running handlers complete
Chef Infra Client failed. 0 resources updated in 03 seconds
===
There was an error running gitlab-ctl reconfigure. Please check the output above for more
details.
===

dpkg: error processing package gitlab-ce (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 gitlab-ce
E: Sub-process /usr/bin/dpkg returned an error code (1)

Related issues

Checklist

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
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Expected (please provide an explanation if not completing)

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