cros lint: Allow unused_ prefix for unused variables.
cros lint: Allow unused_ prefix for unused variables.

The unused_ prefix is allowed by the coding standard. The dummy prefix
is not allowed by the coding standard and has been removed.

BUG=None
TEST=Ran cros lint on a file that uses this prefix.

Change-Id: Ica9eba24eae96b2db7571d4bda5400ea49bda191
Reviewed-on: https://chromium-review.googlesource.com/200197
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Darren Krahn <0c011cef6b0a329cf1a859e1450ec8a1be7f57d9@chromium.org>
Commit-Queue: Darren Krahn <0c011cef6b0a329cf1a859e1450ec8a1be7f57d9@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite