Verification of RTO no longer necessary

[janaiyengar]

The changes to loss recovery in #1974 allow for some simplification in RTO response. The recovery draft says about an ACK after an RTO (in Section 5.3.3):

When an ACK is received that acknowledges only one or more packets sent on an
RTO event, all unacknowledged packets with lower packet numbers MUST be marked
as lost. If packets sent prior to the first RTO are acknowledged in the same ACK, the
RTO is considered spurious and standard loss detection rules apply.

If the ACK acknowledges only packets sent after an RTO, everything sent earlier is marked lost. With time-threshold loss detection in #1974, that is exactly what standard loss detection will do. As a result, in both the conditions described in the text quoted above, standard loss detection does the right thing.

We can remove the two cases (remove OnRetransmissionTimeoutVerified) and always simply use standard loss detection when an ACK is received.

[janaiyengar]

Closed by #2114.