Revert "vp9: Adjust reset segment for real-time screen-content"

Revert "vp9: Adjust reset segment for real-time screen-content"

This reverts commit d72cd51d83947064ef6354f02b1df59d2d3f1317.

Reason for revert: <INSERT REASONING HERE>
Doesn't seem to really remove the artifact that was the cause for this change. Reverting for now.

Original change's description:
> vp9: Adjust reset segment for real-time screen-content
>
> For real-time screen content mode when the short_circuit
> flat_blocks feauture is enabled: reset segment to 0 for
> coding block if its flat, regardless of temporal source_sad.
> Reduces some artifacts on flat areas.
>
> Change-Id: I9620e424bedc5a13f87cc4f66af7c0e86043c89c

TBR=marpan@google.com,builds@webmproject.org,jianj@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I83ee9fd75bfb621a4f3e9afbcc07e7c6ca5c51d6