avcenc: Disable SKIP_BIAS

For static content, intra modes and skip mode compete with each
other neck and neck. From bitstream perspective its favourable to
pick skip. But do not over bias towards skip as intra modes
are already being penalized with mode bits * lambda.

Average bdrates improve by 1%

Test: Tested on clips of different resolutions for quality changes

Change-Id: I8ae85e7e807c9810e10b325627c84eec679327a9