This feature is used for the CBR RTC encoding mode
at speed >= 6. This change will exclude it for VBR mode.

For speed 6 live encoding (VBR):
avgPSNR/SSIM metrics on ytlive set up by ~1% (few clips up by 2/3%).
No change in speed.

Change-Id: I1a0dd94c334f7df309ab5a48d477d7e25355b798