cbuildbot_config: use binary toolchains for arm64

Now that the SDK builds these for us, we can start using prebuilts.

BUG=chromium:421172
TEST=`cbuildbot rush-release` passes
CQ-DEPEND=CL:226685

Change-Id: I75292a702b7daea59ab7e3a3631bd43450cb9f2e
Reviewed-on: https://chromium-review.googlesource.com/228410
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/cbuildbot/cbuildbot_config.py b/cbuildbot/cbuildbot_config.py
index 399cb1b..2f90e58 100755
--- a/cbuildbot/cbuildbot_config.py
+++ b/cbuildbot/cbuildbot_config.py
@@ -1289,7 +1289,6 @@
 chrome_pfq.add_config('rush_ryu-chrome-pfq',
   non_testable_builder,
   boards=['rush_ryu'],
-  usepkg_toolchain=False,
 )
 
 chrome_try = _config(
@@ -2153,7 +2152,6 @@
 
 internal_notest_paladin.add_config('rush_ryu-paladin',
   boards=['rush_ryu'],
-  usepkg_toolchain=False,
   paladin_builder_name='rush_ryu paladin',
 )
 
@@ -2728,7 +2726,6 @@
   non_testable_builder,
   boards=['rush'],
   hw_tests=[],
-  usepkg_toolchain=False,
   # This build doesn't generate signed images, so don't try to release them.
   paygen=False,
   signer_tests=False,
@@ -2739,7 +2736,6 @@
   boards=['rush_ryu'],
   useflags=_release['useflags'] + ['highdpi'],
   hw_tests=[],
-  usepkg_toolchain=False,
 )
 
 ### Per-chipset release groups
diff --git a/cbuildbot/config_dump.json b/cbuildbot/config_dump.json
index 97abac1..1b5a227 100644
--- a/cbuildbot/config_dump.json
+++ b/cbuildbot/config_dump.json
@@ -30067,7 +30067,6 @@
         "useflags": [
             "chrome_internal"
         ],
-        "usepkg_toolchain": false,
         "vm_tests": []
     },
     "rush_ryu-chromium-pfq": {
@@ -30139,7 +30138,6 @@
         "useflags": [
             "chrome_internal"
         ],
-        "usepkg_toolchain": false,
         "vm_tests": []
     },
     "rush_ryu-payloads": {