Remove unused ability to disable flow control in QUIC

Remove unused ability to disable flow control in QUIC

QuicFlowController::Disable() is no longer called. It was used when we
disabled flow control for the headers and crypto streams, and for old
versions which did not support connection level flow control. Now that
we are on QUIC_VERSION_23, we now use flow control on all streams and
all connections, so this is safe to remove.

Thanks to tgreer@ for the scythe code removal in cl/86835556, and
alyssar@ for noticing that the removal of the dead
QuicFlowController:Disable() meant we could remove substantially more in
this CL.

Merge internal change: 86960342

R=rch@chromium.org

[Ryan Hamilton, 2015-03-03 05:01:06]

lgtm