Remove stuff_sound declaration - was never implemented

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9987 387891d4-d844-0410-90c0-e4c51a9137d3
From X3N0-Life-Form: add custom sound for SSM strikes

(I updated the patch to use parse_sound instead of the missing stuff_sound)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9988 387891d4-d844-0410-90c0-e4c51a9137d3