Several packages had documentation about how to use Pigeon or Mockito generation; this made sense when those were rare, but they are both now extremely common in our packages. Instead,
https://github.com/flutter/flutter/pull/158326 adds this information to the general documentation for contribuing to flutter/packages, so it no longer needs to be in (a random subset of) specific packages.

This also removes Contributing sections from several READMEs, as READMEs should be client-focused rather than developer-focused, and puts content in CONTRIBUTING.md where needed.