[blimp] Simplify Blimp buildbot setup.

This CL changes it so that only the minimal configuration needed
is used on the buildbots for blimp.

Unit test targets need to be specified on the test bots they run,
which leads to the target being compiled on the respective builder
bot (there are builder/tester pairs specified in recipes).

Since the blimp_unittests target depends on //blimp:blimp, there
is no need for us to manually specify that target anywhere.

The Android GN (dbg) is special in that it currently has no devices
attached, so it is unable to run any tests.

After this CL, the following builders/testers will run and test
the following targets:
- Android GN: compile (blimp_unittests) + test (blimp_unittests)
- Android GN (dbg): compile (blimp_tests)
- Linux Builder: compile (blimp_unittests)
- Linux Tests: test (blimp_unittests)
- Linux Builder (dbg): compile (blimp_unittests)
- Linux Tests (dbg)(1): test (blimp_unittests)

BUG=543267

Review URL: https://codereview.chromium.org/1408193002

Cr-Commit-Position: refs/heads/master@{#354618}

testing/buildbot/chromium.linux.json

@@ -1,8 +1,6 @@
 {
   "Android GN": {
     "additional_compile_targets": [
-      "blimp",
-      "blimp_tests",
       "chrome_public_apk",
       "chrome_public_test_apk",
       "content_shell_apk",
@@ -76,7 +74,6 @@
   },
   "Android GN (dbg)": {
     "additional_compile_targets": [
-      "blimp",
       "blimp_tests",
       "chrome_public_apk",
       "chrome_public_test_apk",
@@ -659,8 +656,6 @@
   },
   "Linux Tests": {
     "additional_compile_targets": [
-      "blimp",
-      "blimp_tests",
       "mandoline:all"
     ],
     "gtest_tests": [
@@ -1056,8 +1051,6 @@
   },
   "Linux Tests (dbg)(1)": {
     "additional_compile_targets": [
-      "blimp",
-      "blimp_tests",
       "mandoline:all"
     ],
     "gtest_tests": [
@@ -1091,6 +1084,12 @@
         },
         "test": "base_unittests"
       },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "blimp_unittests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true,