[QA] Remove the :core tag since we're not using it anymore

Signed-off-by: Rémy Coutable <remy@rymai.me>

qa/qa/scenario/test/integration/mattermost.rb

@@ -7,7 +7,7 @@ module QA
         # including staging and on-premises installation.
         #
         class Mattermost < Test::Instance::All
-          tags :core, :mattermost
+          tags :mattermost
 
           def perform(address, mattermost, *rspec_options)
             Runtime::Scenario.define(:mattermost_address, mattermost)

qa/qa/specs/features/browser_ui/3_create/repository/create_edit_delete_file_via_web_spec.rb

 # frozen_string_literal: true
 
 module QA
-  context :create, :core do
+  context :create do
     describe 'Files management' do
       it 'user creates, edits and deletes a file via the Web' do
         Runtime::Browser.visit(:gitlab, Page::Main::Login)