Remove bandwidth estimation code.

tinc bandwidth estimation has always been quite unreliable (at least in
my experience), but there's no chance of it working anymore since the
last changes to MTU discovery code, because packets are not sent in
batches of three anymore.

This commit removes the dead code - fortunately, nothing depends on this
estimation (it's not even shown in node info). We probably need be
smarter about this if we do want this estimation back.