Remove best practices now merged into the spec

[emmambd]

A vote to merge specific best practices into the spec has passed. The duplicates should be removed from this repo now.

[isabelle-dr]

The scope for this issue is to remove the Best Practices that will become covered by the spec if the vote passes, correct?

[emmambd]

The goal of this issue is to flag for anyone who might only be paying attention to https://github.com/MobilityData/GTFS_Schedule_Best-Practices to go to google/transit#386

I'll update this issue if the vote passes so it focuses on removing the best practices covered by spec.