Remove isolate server recipe property

Isolate server is not used anymore.

Bug: webrtc:12072
Change-Id: Ibd697ea1d3ea45623e047485a8856e4c0e1e557a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272870
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37900}

infra/config/config.star

@@ -477,9 +477,6 @@ def webrtc_builder(
       A luci.builder.
     """
     properties = properties or {}
-    properties["$recipe_engine/isolated"] = {
-        "server": "https://isolateserver.appspot.com",
-    }
     resultdb_bq_table = "webrtc-ci.resultdb." + bucket + "_test_results"
     return luci.builder(
         name = name,