Drop prebuilts/boot-artifacts

As it's not maintained any more, according to the commit here[1],
drop it to save disk spaces

[1]: https://android.googlesource.com/platform/prebuilts/boot-artifacts/+/45d30d07ad284018481ebb419b5d01b5de72ed02

Test: build android-mainline gki kernel

Change-Id: Iad2a00ddb018cdebaa937e1610801a55253b9bb6
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
(cherry picked from commit 70e20f75939c80fd304d6e72f73ee3899f63a611)
(cherry picked from commit a727f4d1e16c4411b6a583c6074fca6868b726b1)
1 file changed
tree: 136d22a65d144c14dc8b017ea43e15205be8ec5a
  1. default.xml