Remove Linux ChromiumOS GN build configs from //testing/buildbot.

Since the builders are being removed.

R=dnj@chromium.org
BUG=649579

Review-Url: https://codereview.chromium.org/2358133005
Cr-Commit-Position: refs/heads/master@{#420727}

testing/buildbot/chromium.chromiumos.json

@@ -4,62 +4,6 @@
       "All"
     ]
   },
-  "Linux ChromiumOS GN": {
-    "additional_compile_targets": [
-      "gn_all"
-    ],
-    "gtest_tests": [
-      {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "base_unittests"
-      },
-      {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "exo_unittests"
-      },
-      {
-        "args": [
-          "--override-use-gl-with-osmesa-for-tests"
-        ],
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "mash_unittests"
-      }
-    ]
-  },
-  "Linux ChromiumOS GN (dbg)": {
-    "additional_compile_targets": [
-      "gn_all"
-    ],
-    "gtest_tests": [
-      {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "base_unittests"
-      },
-      {
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "exo_unittests"
-      },
-      {
-        "args": [
-          "--override-use-gl-with-osmesa-for-tests"
-        ],
-        "swarming": {
-          "can_use_on_swarming_builders": true
-        },
-        "test": "mash_unittests"
-      }
-    ]
-  },
   "Linux ChromiumOS Ozone Tests (1)": {
     "gtest_tests": [
       {

testing/buildbot/gn_isolate_map.pyl

@@ -462,10 +462,6 @@
     "type": "raw",
     "args": [],
   },
-  "gn_all": {
-    "label": "//:gn_all",
-    "type": "additional_compile_target",
-  },
   "gn_unittests": {
     "label": "//tools/gn:gn_unittests",
     "type": "raw",