Fix "Unable to resolve dependency" error during forum init

When running "bundle exec rake search:initialize" in the forum, we were
getting the following error:

        /openedx/ruby/lib/ruby/site_ruby/2.4.0/rubygems/resolver.rb:235:in `search_for': Unable to resolve dependency: user requested 'did_you_mean (= 1.1.0)' (Gem::UnsatisfiableDependencyError)

It happens this error suddently happened because rubygems-update was not
pinned to a specific version. v3.0.4 was working and v3.1.2 was not.

As it happens, we don't need rubygems-update, so we simply get rid of it
entirely.

CHANGELOG.md

@@ -4,6 +4,7 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix "Unable to resolve dependency" error during forum initialisation
 - [Feature] Add `settheme` command to easily assign a theme to a domain name
 - [Improvement] Modify nginx access logs to include request scheme and server name (plugin developers should use the "tutor" log format)
 - [Bugfix] Fix DNS resolution of restarted service

tutor/templates/build/forum/Dockerfile

@@ -26,8 +26,6 @@ RUN ruby-build $RUBY_VERSION /openedx/ruby
 ENV PATH "/openedx/ruby/bin:$PATH"
 RUN gem install bundler -v $BUNDLER_VERSION
 RUN gem install rake -v $RAKE_VERSION
-# gem upgrade must come after bundler/rake install
-RUN gem install rubygems-update && update_rubygems
 
 # Install forum
 RUN git clone https://github.com/edx/cs_comments_service.git --branch open-release/ironwood.2 --depth 1 /openedx/cs_comments_service