Resolve "[AB Test] Remove Talk to Sales test code"

Step 1: What is changing in this MR?

Reverts the AB test code added in this MR. The winner was the variant (Talk to Sales).

• Turn off the test in Launch Darkly
• Update contentful navigation to have the winning button
• Delete the unused code (this MR)
• Delete the duplicate navigation Contentful entry (should be done after merging this MR)

Production	Review app
https://about.gitlab.com/pricing/	https://3603-ab-test-remove.about.gitlab-review.app/pricing/

Step 2: Ensure that your changes comply with the following, where applicable:

• I, the Assignee, have run Axe tools on any updated pages, and fixed the relevant accessibility issues.
• These changes work on both Safari, Chrome, and Firefox.
• These changes have been reviewed for Visual Quality Assurance and Functional Quality Assurance on Mobile, Desktop, and Tablet.
• These changes work with our Google Analytics and SEO tools.
• These changes have been documented as expected.

Build Variables:

• Use Contentful Preview API

Closes #3603 (closed)