[milo] remove linkBase and calculate links directly.

milo/buildsource/swarming/expectations/build-gerrit.json

 {
   "Summary": {
     "Label": "2d8d62f6be545b10",
     "Status": "Running",
     "Source": {
       "Label": "Task 2d8d62f6be545b10",
       "URL": "https://example.com/task?id=2d8d62f6be545b10\u0026show_raw=1\u0026wide_logs=true"
     },
     "MainLink": [
       {
         "Label": "Task 2d8d62f6be545b10",
         "URL": "https://example.com/task?id=2d8d62f6be545b10\u0026show_raw=1\u0026wide_logs=true"
       }
     ],
     "SubLink": [
       [
         {
           "Label": "stdout",
-          "URL": "foo/stdout"
+          "URL": "/swarming/task/build-gerrit/stdout"
         }
       ]
     ],
     "Started": "2016-03-14T10:00:01.85071Z",
     "Finished": "0001-01-01T00:00:00Z",
     "Duration": 3598149290000,
     "Type": "Recipe",
     "LevelsDeep": 0,
     "Verbosity": 1,
     "Text": [
@@ skipping 18 matching lines @@
     "File": null,
     "Source": ""
   },
   "Components": [
     {
       "Label": "recipe bootstrap",
       "Status": "Running",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/recipe_bootstrap/0/stdout"
+          "URL": "/swarming/task/build-gerrit/steps/recipe_bootstrap/0/stdout"
         }
       ],
       "SubLink": [
         [
           {
             "Label": "Confused? README here",
             "URL": "https://chromium.googlesource.com/external/github.com/luci/recipes-py/+/master/doc/workflow.md"
           }
         ]
       ],
@@ skipping 11 matching lines @@
       "GroupName": "recipe bootstrap",
       "Property": null
     },
     {
       "GroupName": "Main",
       "Property": null
     }
   ],
   "Blame": null
 }