Stop using ios-isolated-tester@ on chromium-based ios bots.

Bug: 793982
Change-Id: I35c4843cd0daa4987c4384e093147e08e719a30a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2121077Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753379}

testing/buildbot/mixins.pyl

@@ -267,11 +267,6 @@
       ],
     },
   },
-  'ios_swarming_account': {
-    'swarming': {
-      'service_account': 'ios-isolated-tester@chops-service-accounts.iam.gserviceaccount.com'
-    },
-  },
   'kitkat': {
     'swarming': {
       'dimensions': {

testing/buildbot/waterfalls.pyl

@@ -1866,7 +1866,6 @@
       'ios-simulator-code-coverage': {
         'mixins': [
           'code-coverage',
-          'ios_swarming_account',
           'mac_10.14',
           'mac_toolchain',
           'out_dir_arg',
@@ -1878,7 +1877,6 @@
       },
       'ios-simulator-cronet': {
         'mixins': [
-          'ios_swarming_account',
           'mac_10.14',
           'mac_toolchain',
           'out_dir_arg',
@@ -1891,7 +1889,6 @@
       'ios-webkit-tot': {
         'mixins': [
           'ios_custom_webkit',
-          'ios_swarming_account',
           'mac_10.14',
           'mac_toolchain',
           'out_dir_arg',
@@ -1906,7 +1903,6 @@
           'all',
         ],
         'mixins': [
-          'ios_swarming_account',
           'mac_10.14',
           'mac_toolchain',
           'out_dir_arg',
@@ -1926,7 +1922,6 @@
           'all',
         ],
         'mixins': [
-          'ios_swarming_account',
           'mac_10.14',
           'mac_toolchain',
           'out_dir_arg',