Disable KeySilkCases and GPURasterSet

Disable them till the telemetry rev in CT is updated and these page sets are
directly invoked from chromium/src/tools/perf/page_sets directory instead of
maintaining a copy of them in CT's datastores.

BUG=skia:3140
TBR=borenet

Review URL: https://codereview.chromium.org/742073002