[build] Remove python version detection

When 2.14 hits stable, all branches will have been migrated to python3.

Change-Id: Ic652ef7f1a5d4f879d90ce76426795a8ac9db204
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/212564
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
35 files changed