Update mac_ios tests to run on either arm64 or x86 (flutter#168341)

We have more arm64 bots than x86, but most of our tests are on
`mac_ios`, which is filtered to `x86`. This PR removes that filter and
allows tests to run on either arch.

Fixes flutter#168236.

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Author: vashworth

.ci.yaml

@@ -285,7 +285,6 @@ platform_properties:
           {"dependency": "apple_signing", "version": "version:to_2025"}
         ]
       os: Mac-14
-      cpu: x86
       device_os: iOS-17|iOS-18
       $flutter/osx_sdk : >-
         {