[SCReAMv2] Inline reference window backoff calculation due to delay in ScreamV2

Move the reference window backoff calculation from DelayBasedCongestionControl
to ScreamV2 under the else-if (is_virtual_ce) branch. This enables ScreamV2
to manage reference window changes centrally, updating it exactly once per feedback.

- Removed min_delay_based_bwe_ limit and DelayBasedCongestionControl::UpdateReferenceWindow.
- Simplified backoff branches in ScreamV2 using a single backoff variable applied once at the end.
- Structured the backoff checks as a mutually exclusive else-if chain (loss first, ECN-CE second, delay third).
- Cleaned up obsolete unit tests from delay_based_congestion_control_unittest.cc.

Bug: webrtc:447037083
Change-Id: Ied3ea4eb49da7d576feb75970ec2eb55994b6e12
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/476742
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47863}