Remove redundant file: testing/buildbot/chromium_arm.json

This was used by the legacy chromium ARM builder and
is not longer needed (AFAICT).

Review URL: https://codereview.chromium.org/1536123003

Cr-Commit-Position: refs/heads/master@{#367178}

testing/buildbot/chromium_arm.json

-{
-  "compile_targets": [
-    "browser_tests_run",
-    "nacl_helper_nonsfi_unittests_run",
-    "nacl_loader_unittests_run",
-    "sandbox_linux_unittests_run"
-  ],
-  "gtest_tests": [
-    "browser_tests",
-    "nacl_helper_nonsfi_unittests",
-    "nacl_loader_unittests",
-    "sandbox_linux_unittests"
-  ]
-}