infra: Stop statically enforcing Xcode version.

The OSX machines in "luci.flex.try" pool that run Dawn tryjobs
have been upgraded from OSX 10.14 to OSX 10.15. Per [1],
the preferred Xcode version on OSX 10.15 is '12a7209' and most
tryjobs (from other projects) install that version. But when
a Dawn tryjob runs, it rolls Xcode back to '11b52'. Reinstalling Xcode
back and forth like that takes time (~20 min), it caused some tryjobs
in the "infra" project to timeout.

Unhardcode Xcode version in Dawn configs. It should allow the recipe
to pick the "best" version ('12a7209' in this case).

[1]: https://source.chromium.org/chromium/chromium/tools/depot_tools/+/master:recipes/recipe_modules/osx_sdk/api.py;l=21?q=11b52&ss=chromium

R=cwallez@chromium.org, tandrii@chromium.org

Change-Id: I8499a763679b9e9270854bf66c2a21f1e6354594
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/29440
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

infra/config/global/cr-buildbucket.cfg

@@ -104,11 +104,6 @@ builder_mixins {
     path: "osx_sdk"
   }
   recipe {
-    properties_j: <<EOF
-      $depot_tools/osx_sdk: {
-        "sdk_version": "11b52"
-      }
-    EOF
     properties_j: <<EOF
       $build/goma: {
         "rpc_extra_params": "?prod",