[testing] remove unused mixins

Change-Id: Ie171df6c108992ecaf417f95de24bb43e1d79967
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732076
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683504}

testing/buildbot/chromium.linux.json

@@ -2151,8 +2151,7 @@
       {
         "isolate_coverage_data": true,
         "name": "check_network_annotations",
-        "script": "check_network_annotations.py",
-        "swarming": {}
+        "script": "check_network_annotations.py"
       }
     ]
   },

testing/buildbot/waterfalls.pyl

@@ -3331,7 +3331,6 @@
       'Linux Builder': {
         'mixins': [
           'code-coverage',
-          'linux-trusty',
         ],
         'additional_compile_targets': [
           'all'