Stop running module_test_ios in devicelab and x64 Macs

[jmagman]

As of #147934 module_test_ios is passing on non-devicelab machines. Remove the devicelab build and mark the arm non-devicelab build as not flaky.

@vashworth's summary from #112033 (comment):

So I believe the idea was to move Mac_arm64_ios module_test_ios to Mac_arm64 module_test_ios so it doesn't run on a devicelab bot.

However, it fails when running on a chromium bot due to a Trying to load an unsigned library issue, even though it's using a simulator:
https://ci.chromium.org/ui/p/flutter/builders/luci.flutter.staging/Mac_arm64%20module_test_ios

It's unclear why, but it seemed that removing the lines that force disabling of codesigning allowed it to run. @godofredoc was going to do a follow up PR to fix it, but it was probably forgotten.

Also remove the x64 version of this test, since there's nothing arch-specific going on here and it happens to be flaky due to an infrastructure issues.

Fixes #112033
Fixes #144680

[vashworth]

Do we also want to remove Mac_x64 module_test_ios, do we need to running on both arch?

[jmagman]

Do we also want to remove Mac_x64 module_test_ios, do we need to running on both arch?

Good idea. That one is my fault, when I wrote go/flutter-mac-test-architectures I put anything testing the tool as needing to run on both archs, but we can be more conservative than that.

[dcharkes]

FYI:
In my experience, running on simulators doesn't require us to get the signing right, but running on device does.
Signing for native assets should also be covered in the native assets integration test, and it's unlikely that we'll do signing differently in the code path for native assets in add2app, so it's probably safe to remove coverage for signing in the add2app scenario.

[vashworth]

LGTM!