Remove mocks from web devices test

[jmagman]

Remove mocks from web devices_test.dart and a few other unrelated unused Mock classes.

Delete Edge device is not listed when Edge cannot be run.

1. This wasn't testing any Edge device case because the test platform was Linux. So the test was passing because _edgeDevice was always null, not because an executable was unrunnable.
2. When I tried to fix the test, there's nothing related to Edge devices not being found due to some executable being unrunnable. I fixed the test to find Edge devices, and it never calls canRun anywhere. It only does a version check to decide whether to show the Edge device, which is covered in another test
   

   flutter/packages/flutter_tools/lib/src/web/web_device.dart

   Lines 354 to 355 in 021311e

   	if (await _edgeDevice?._meetsVersionConstraint() ?? false)
   	_edgeDevice,

   
   

   flutter/packages/flutter_tools/lib/src/web/web_device.dart

   Lines 259 to 266 in 021311e

   	Future<bool> _meetsVersionConstraint() async {
   	final String rawVersion = (await sdkNameAndVersion).replaceFirst('Microsoft Edge ', '');
   	final Version version = Version.parse(rawVersion);
   	if (version == null) {
   	return false;
   	}
   	return version.major >= _kFirstChromiumEdgeMajorVersion;
   	}

Part of #71511

[jonahwilliams]

LGTM!