rely on Irssi::pidwait_add() and don't reap anything manually

Switch to proposed 'variant 3':
Irssi::pidwait_add() already does a waitpid() for any child.
Manual calls to waitpid() and Irssi::pidwait_remove() should not be
necessary, so remove them altogether.

(I've grep(1)ed the irssi scripts directory on my Debian stable and
 none of the scripts (excepit twirssi :-) that calls pidwait_add()
 calls either pidwait_remove() or waitpid(), so this should work.)