deprecate GBM_BO_USE_WRITE

deprecate GBM_BO_USE_WRITE

There are no users of this flag, and the only thing it is defined to
affect is gbm_bo_write, which has no implementation. I suspect this is a
leftover from the original gbm that is no longer needed.

This also removes the declaration of gbm_bo_write to keep others from
thinking minigbm actually implements it.

BUG=None
TEST=minigbm still builds

Change-Id: I0b72efa14208b32e924292cef957273359c7d40b
Reviewed-on: https://chromium-review.googlesource.com/331289
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>