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)
diff --git a/default.xml b/default.xml
index 4399e30..768d8d8 100644
--- a/default.xml
+++ b/default.xml
@@ -27,7 +27,6 @@
   <project path="common-modules/virtual-device" name="kernel/common-modules/virtual-device" revision="android13-5.15" />
   <project path="prebuilts/clang/host/linux-x86" name="platform/prebuilts/clang/host/linux-x86" clone-depth="1" />
   <project path="prebuilts/gcc/linux-x86/host/x86_64-linux-glibc2.17-4.8" name="platform/prebuilts/gcc/linux-x86/host/x86_64-linux-glibc2.17-4.8" clone-depth="1" />
-  <project path="prebuilts/boot-artifacts" name="platform/prebuilts/boot-artifacts" />
   <project path="prebuilts/build-tools" name="platform/prebuilts/build-tools" clone-depth="1" />
   <project path="prebuilts/kernel-build-tools" name="kernel/prebuilts/build-tools" clone-depth="1" />
   <project path="tools/mkbootimg" name="platform/system/tools/mkbootimg" />