remove replay viewer text label

This label didn't seem to contribute much of anything, and it
caused big problems for translators because we don't have
dynamically resizing text labels, and that would have provoked
intractable layout problems anyways, so we decide just to remove
it and rely on the tooltips / media controller icons on the buttons
to tell the user what is going on.

See also discussion here:
http://forums.wesnoth.org/viewtopic.php?f=9&t=24598&start=120#p575851