This covers one of the most basic commands in Linux, I suggest you dig into any Linux tutorial or introduction to become familiar with commands like mkdir, ls, ps, rm, mv, cp, ...: See for example the Rute tutorial:
http://rute.2038bug.com/index.html.gz
If you have nothing mounted at that mount point (don't know what happens else) you do either a "rmdir /mnt/tmp" which will remove the folder if it is empty or "rm -r /mnt/tmp" which will remove the folder and all files in it recursively, which you should only do if you are sure about that. Try rmdir first.
