[System] Remove any CFNetwork usage from the watchOS profile. Fixes #45847.

[System] Remove any CFNetwork usage from the watchOS profile. Fixes #45847.

The MacProxy class uses CFNetwork, but since CFNetwork is not a public
framework on watchOS, we can't use it.

So remove MacProxy completely (it only contains internal classes), and throw
PlatformNotSupportedException in any API that used it (the managed networking
stack is not supported on watchOS anyway, so this should be safe).

https://bugzilla.xamarin.com/show_bug.cgi?id=45847