If you don't need to use the MySQL server, you can simply prevent it from starting up. This should make the port available to other software.
To disable MySQL from starting up, go to /Library/Startup Items and either archive or rename the MySQL item(s) there. Then reboot.
Otherwise, if you really want to get rid of it altogether, remove the startup items and remove the MySQL folder in /usr/local/. If you're not comfortable with the terminal, choose "Go to Folder" from the "Go" menu in Finder and type in /usr/local and press return. The Finder will open that folder and you can then delete the MySQL folders.
Another option would be to change the port number used by one of the two items. Perhaps Xampp has a preference that allows you to change the port number. If not, you can change MySQL's port number by downloading MySQL administrator software (free) from the MySQL.org web site: http://www.mysql.org/downloads/gui-tools/5.0.html