vp9: Fix to quality regression issue for 1 pass.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1217

Issue introduced in https://chromium-review.googlesource.com/#/c/339162/.
The factor on avg_frame_qindex[INTER_FRAME] in that CL, used for q-basis to
set active_best, can cause the QP to decrease too slowly or get stuck
in some cases when max-q=63.

Removing that factor fixes the regression in issue#1217.

Change-Id: I97fe28dca3c2bc2d852b1775f39c62164e032d4f