update_gpu_drivers: Remove call to equery-${BOARD}

The next CL will fetch the prebuilt from Google Storage, and allow
updating the prebuilt without having to run setup_board for the
target ${BOARD}.

Remove code that relies on equery-${BOARD}:
 - ebuild category is always media-libs
 - current binary version can be obtained with a simple sed
   expression

BUG=chromium:739235
TEST=./update_gpu_drivers.sh -n --package mali-drivers-bifrost

Change-Id: I2f5c20b24fdcdd4073a2cb17699dfbec77f0d059
Reviewed-on: https://chromium-review.googlesource.com/1350451
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed