As part of #99885, the LUCI recipes for flutter must be updated to only run tests for Safari Desktop instead of iOS Safari. Since flutter/engine#33757 has merged, we can now validate golden testing with Webkit via Desktop Safari instead of iOS, which removes the dependency on the iOS Simulator and Safari, which has been a source of flakiness on newer builds of macOS.

Looking through the recipe, a few things that might need updating:

1. The recipe actually installs Xcode twice per run. I believe that is just an error in the recipe itself, but after this change I think we should be able to get away with not installing Xcode at all for the web_engine tests, since we only installed Xcode for the iOS simulator previously.
2. macOS has some special logic to compile the tests in a separate test, so that the same compiled tests can be used for iOS and macOS Safari. This is no longer necessary since we will be running on only one browser
3. There is an additionalArgs variable in the recipe that is only updated in the macOS branch, but only used in a non-macOS path. As a result I think this variable is actually unnecessary and can just be removed.

Once the recipe is updated, we can make a small change to the .ci.yaml in the engine repo to remove the dependency on old Xcode/macOS. Once that lands, the build bots running older macOS versions can be upgraded.