Remove adb as something buildbots need to build.

adb is getting a framework dependency with:
https://android-review.googlesource.com/c/platform/system/core/+/756410

Which we don't want to include.

Test: buildbot-build.sh
Change-Id: I923e2026c5857cef7118aa21f357fcabbc54d6fc
diff --git a/tools/buildbot-build.sh b/tools/buildbot-build.sh
index b5d39e1..3d70087 100755
--- a/tools/buildbot-build.sh
+++ b/tools/buildbot-build.sh
@@ -75,7 +75,7 @@
   make_command="make $j_arg $extra_args $showcommands build-art-target-tests $common_targets"
   make_command+=" libjavacrypto-target libnetd_client-target linker toybox toolbox sh"
   make_command+=" debuggerd su"
-  make_command+=" ${out_dir}/host/linux-x86/bin/adb libstdc++ "
+  make_command+=" libstdc++ "
   make_command+=" ${ANDROID_PRODUCT_OUT#"${ANDROID_BUILD_TOP}/"}/system/etc/public.libraries.txt"
   if [[ -n "$ART_TEST_CHROOT" ]]; then
     # These targets are needed for the chroot environment.