Remove deprecation warning for rbenv -> asdf

This change removes the deprecation warning and shim we added during the
transition from rbenv to asdf.

zsh/configs/post/path.zsh

@@ -4,16 +4,6 @@ PATH="$HOME/.bin:/usr/local/sbin:$PATH"
 # load ASDF, falling back to rbenv if not available
 if [ -d "$HOME/.asdf" ]; then
   . $HOME/.asdf/asdf.sh
-elif command -v rbenv >/dev/null; then
-  if [ -z $SILENCE_RBENV_DEPRECATION ]; then
-    echo "The thoughtbot dotfiles have deprecated the use of rbenv in favor"\
-         "of asdf (https://github.com/asdf-vm/asdf) and will be removed on or"\
-         "after December 8, 2017.  Migrate to asdf or export"\
-         "SILENCE_RBENV_DEPRECATION=1 in your local dotfiles to silence this"\
-         "deprecation."
-  fi
-
-  eval "$(rbenv init - --no-rehash)"
 fi
 
 # mkdir .git/safe in the root of repositories you trust