Remove Unused move_to_front_of_path Function

In #440, a `move_to_front_of_path` function was introduced, but
was [ultimately not used][1] when these changes were merged as f7c73f7.
The function itself was not removed, however.

This commit removes the unused function.

[1]: https://github.com/thoughtbot/dotfiles/pull/440#issuecomment-156549420

zsh/functions/move_to_front_of_path

@@ -1,10 +0,0 @@
-# Reorders existing entry in PATH to have it at the front
-#
-#   move_to_front_of_path "$HOME/.bin"
-#
-move_to_front_of_path() {
-  if [[ ":$PATH:" == *":$1:"* ]]; then
-    PATH=$(echo $PATH | sed 's#'$1'##g' | sed s/:://g | sed s/:$//g | sed s/^://g)
-    PATH="$1${PATH:+":$PATH"}"
-  fi
-}