pip 2020 resolver no longer valid

[chrisjkuch]

Attempting to run a build on the latest v2 branch yields the following:

option --use-feature: invalid choice: '2020-resolver' (choose from 'fast-deps', 'truststore', 'no-binary-enable-wheel-cache')

Pip 22.3 removes that option since this resolver is now default.

[jayqi]

I don't think we have this anywhere anymore.