[iOS][infra] Remove iOS14 tester pool "chromium.tests.ios14" config.

The pool is going to be removed.

Bug: 1119885
Change-Id: I5dc5511a8078e305a139c07fd572ab2fac373fe6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2378777Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802373}

testing/buildbot/mixins.pyl

@@ -207,13 +207,6 @@
       'service_account': 'chromium-tester@chops-service-accounts.iam.gserviceaccount.com',
     },
   },
-  'chromium-tests-ios14': {
-    'swarming': {
-      'dimensions': {
-        'pool': 'chromium.tests.ios14',
-      },
-    },
-  },
   'dawn_end2end_gpu_test': {
     '$mixin_append': {
       'args': [

testing/buildbot/waterfalls.pyl

@@ -2149,7 +2149,6 @@
           'all',
         ],
         'mixins': [
-          'chromium-tests-ios14',
           'mac_10.15',
           'mac_toolchain',
           'out_dir_arg',
@@ -2166,7 +2165,6 @@
           'all',
         ],
         'mixins': [
-          'chromium-tests-ios14',
           'mac_10.15',
           'mac_toolchain',
           'out_dir_arg',