V8 Buildbot: Remove swarming staging builders/testers.

scripts/slave/gatekeeper.json

 {
   "comment": ["This is a configuration file for gatekeeper_ng.py",
               "Look at that for documentation on this file's format."],
 
   "masters": {
     "https://build.chromium.org/p/chromium": [
       {
         "categories": [
           "chromium_tree_closer"
         ],
@@ skipping 514 matching lines @@
             "tsepez@chromium.org"
         ]
       }
     ],
     "https://build.chromium.org/p/client.v8": [
       {
         "categories": [
           "v8_all"
         ],
         "excluded_builders" : [
-          "V8 Linux - swarming staging builder",
-          "V8 Linux - swarming staging 1",
-          "V8 Linux - swarming staging 2",
-          "V8 Linux - swarming staging 3",
+          "V8 Linux - swarming staging",
           "V8 Linux - debug - greedy allocator",
           "V8 Linux64 - debug - greedy allocator",
           "V8 Linux64 - cfi",
           "V8 Linux64 - gcov coverage",
           "V8 Linux - mipsel - sim",
           "V8 Linux - ppc - sim",
           "V8 Linux - ppc64 - sim",
           "V8 Linux - x87 - nosnap - debug builder",
           "V8 Linux - x87 - nosnap - debug",
           "V8 Linux - full debug",
@@ skipping 366 matching lines @@
         "cleanup_temp",
         "taskkill"
       ],
       "closing_optional": [
         "compile",
         "gn"
       ]
     }
   }
 }