De-quarantine plan end-to-end test

I have analyzed the execution of this test in the quarntine jobs
of some MRs, and in all of the executions that I looked it has
passed, so I'm de-quarantining it, but I'll keep an eye on it
in case it fails again.

qa/qa/specs/features/browser_ui/2_plan/issue/filter_issue_comments_spec.rb

 # frozen_string_literal: true
 
 module QA
-  # Failure issue https://gitlab.com/gitlab-org/quality/staging/issues/68
-  context 'Plan', :quarantine do
+  context 'Plan' do
     describe 'filter issue comments activities' do
       let(:issue_title) { 'issue title' }