extend PATH in pam_environment instead of zsh config

This way, the extended PATH variable is available for the whole
session and not just for the zsh session.

zsh/exports.zsh

@@ -2,10 +2,3 @@
 if type vim >/dev/null 2>&1; then
 	export EDITOR="vim"
 fi
-
-# extend PATH variable
-export PATH="/usr/local/sbin:/usr/sbin:/sbin:$PATH"
-
-if [ -d "$HOME/bin" ]; then
-	export PATH="$HOME/bin:$PATH"
-fi