Revert "DEV: Parse and install bundler version from Gemfile.lock (#98)"

This reverts commit 25fc00a763.

This causes a RuntimeError during upgrade, because the `discourse` user does not have permission to install system gems

```
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /usr/local/lib/ruby/gems/2.7.0 directory.
Docker Manager: FAILED TO UPGRADE
```

lib/docker_manager/upgrader.rb

@@ -90,10 +90,6 @@ class DockerManager::Upgrader
       percent(20 * (index + 1) / @repos.size)
     end
 
-    bundler_version = parse_bundler_version
-    raise "Unable to determine bundler version from lockfile" if bundler_version.nil?
-    run("gem install bundler --conservative -v #{bundler_version}")
-
     run("bundle install --deployment --jobs 4 --without test development")
     begin
       run("bundle exec rake plugin:pull_compatible_all")
@@ -243,12 +239,6 @@ class DockerManager::Upgrader
 
   private
 
-  def parse_bundler_version
-    File.read("Gemfile.lock").lines.each_cons(2).find do |heading, version|
-      return version.strip if heading.strip == "BUNDLED WITH"
-    end
-  end
-
   def pid_exists?(pid)
     Process.getpgid(pid)
   rescue Errno::ESRCH