Remove po alias

https://github.com/robbyrussell/oh-my-zsh/issues/6761

lib/directories.zsh

@@ -28,7 +28,3 @@ alias lsa='ls -lah'
 alias l='ls -lah'
 alias ll='ls -lh'
 alias la='ls -lAh'
-
-# Push and pop directories on directory stack
-alias pu='pushd'
-alias po='popd'