PRR: Avoid accounting left-edge twice in partial ACK.

[rscheff]

Summary

After D18624, accounting the advance of the left edge
in a partial ack is already included in
sackhint.delivered_data.

Therefore this is a cumulative error of the
prr_delivered bytes, over the course of multiple
partial acks while lost packets get acknowledged in
sequence.

Test Plan

Corrected behavior, maximum of 2 segments per ACK:

prr-partialack-fixed.pkt3 KBDownload

Incorrect behavior due to double-accounting (counterintutively, being more agressive pushing data out - 3 segments per ACK):

prr-partialack.pkt3 KBDownload

[rscheff]

• rebase to HEAD