[keyserver] Remove Python2 requirement from Docker

Summary:
After bumping the sqlite3 dependency, Python2 is no longer needed when building SQLite from source.

The dependency on Python2, as well as any steps to have python resolve to Python2 can now be removed.

Linear issue: https://linear.app/comm/issue/ENG-1157/remove-python2-requirement-from-docker

Depends on D4103

Test Plan:

1. arc patch D4103
2. yarn cleaninstall
3. cd keyserver && docker-compose up --build
4. Ensure that the yarn cleaninstall step "within" the container succeeds.

Reviewers: palys-swm, ashoat, def-au1t

Reviewed By: ashoat

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4106