Part 3: Abstract pipeline_variables_form to its own component - clean up

What does this MR do and why?

Part 3 for #520831 (closed)

We are abstracting the pipeline variables form to it's own component in order to accommodate inputs work and also hide the component depending on settings.

Part 3 removes the redundant pipeline variables code

Step	MR
Set up component/query	!182376 (merged)
Build out pipeline variable form component	!182455 (merged)
Clean up and integration	[Current MR]

Screenshots or screen recordings

No UI change

How to set up and validate locally

1. Navigate to Build > Pipeline editor and enter the following into your gitlab-ci.yml

variables:
  DEPLOY_ENVIRONMENT:
    value: "staging"
    options:
      - "production"
      - "staging"
      - "canary"
    description: "The deployment target. Set to 'staging' by default."

job:
  script: echo 1

2. Navigate to -/pipelines/new and verify that your variables are output in the pipeline variables form.
3. update variables and click "New pipeline"

Related to #520831 (closed)

app/assets/javascripts/ci/job_details/components/sidebar/jobs_container.vue

@@ -25,7 +25,9 @@ export default {
 </script>
 <template>
   <div class="block builds-container">
-    <b class="gl-mb-2 gl-flex gl-font-semibold">{{ __('Related jobs') }}</b>
+    <h2 class="gl-m-0 gl-mb-2 gl-text-base gl-leading-reset">
+      {{ __('Related jobs') }}
+    </h2>
     <job-container-item
       v-for="job in jobs"
       :key="job.id"