Removed some more peripheral dead code.

The PerSetButtonMapping callback was added seven
years ago and was never used, guess it's ok to
remove it :)
Also removed the PerRemovePeripheral function that
was never implemented.

git-svn-id: https://yabause.svn.sourceforge.net/svnroot/yabause/trunk@3120 d7be59e3-070e-4b9a-85a1-4646befae34d