Final call: remove techniques from text-contrast acc mapping

Hey folks,
One more! This one removes two techniques from the accessibility
requirements mapping from the text-contrast rule. If you're curious why, an
explanation is here:
https://github.com/act-rules/act-rules.github.io/pull/1408#issuecomment-674813471

This final call ends on August 26th. If you have any feedback, you can
leave a review on the pull request:
https://github.com/act-rules/act-rules.github.io/pull/1408

-- 
*Wilco Fiers*
Axe for Web product owner - Co-facilitator WCAG-ACT - Chair ACT-R