Remove old, deprecated code

[samuelcolvin]

Change Summary

Remove the following deprecated functions (all have had deprecation warnings for multiple releases):

• get_validators is no longer recognised, use __get_validators__
• Config.ignore_extra is no longer recognised, use Config.extra
• Config.allow_extra is no longer recognised, use Config.extra

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• change description file added to changes/,
  see changes/README.md for details
  on the format

[codecov]

Codecov Report

Merging #720 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #720      +/-   ##
==========================================
- Coverage     100%   99.92%   -0.08%     
==========================================
  Files          15       15              
  Lines        2727     2708      -19     
  Branches      539      532       -7     
==========================================
- Hits         2727     2706      -21     
- Misses          0        2       +2