Fix pipeline 1011175 due to removed learn about from free plan on billings page

Revert MR: !1409 (merged)

What does this MR do and why?

Remove override of the purchase_link method.

Upstream MR：https://gitlab.com/gitlab-org/gitlab/-/commit/bdac9a33430067f50fe695486eb35fe55cbb7bb6

In Refactor free plan billing page (!1448 - merged), we need to override annual_price_text and plans_data in PlanComponent, so I don’t remove it.

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