Remove XP and Vista bots from chromium.win and chromium.webkit waterfalls.

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 353 matching lines @@
             "WebKit Linux Oilpan (dbg)",
             "WebKit Linux Oilpan",
             "WebKit Linux Oilpan Leak",
             "WebKit Linux Oilpan ASAN",
             "WebKit Mac Builder (deps)",
             "WebKit Mac Oilpan (dbg)",
             "WebKit Mac Oilpan",
             "WebKit Mac10.6 (deps)",
             "WebKit Win Builder (deps)",
             "WebKit Win Oilpan (dbg)",
-            "WebKit Win Oilpan",
-            "WebKit XP (deps)"
+            "WebKit Win Oilpan"
         ],
 
         "builders": {
           "*": {
             "categories": [
               "chromium_tree_closer"
             ]
           }
         }
       }
@@ skipping 488 matching lines @@
         "cleanup_temp",
         "taskkill"
       ],
       "closing_optional": [
         "compile",
         "gn"
       ]
     }
   }
 }