Change alias from rmwd to rmsubdir

The longer alias makes it more clear what's happening. It's longer than
rmwd but it can be accelerated with tab completion. "rms<TAB>" gives
four characters to remove a subdirectory vs. "rm -rf" which is six,
which I'm not sure is an improvement.  rmsubdir is weaker though since
it only targets files in the current directory.
Ensure only directories and symlinks are removed