[System][ios] Disable non-working Ping code (and non-existing DllImport)
* capget is not part of libc (libSystem.dylib) on iOS (or OSX); and
* There's no working Process class to invoke `ping` by itself leading to
[FAIL] DllImportTest.LackOfCapget : System.InvalidOperationException : Process has not been started.
at System.Diagnostics.Process.get_HasExited () [0x00039] in /Users/poupou/git/xamarin/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/System/System.Diagnostics/Process.cs:184
It's technically possible to implement ping on iOS [1] so the code is
not totally removed from the build (API remains identical to existing).
[1] https://bugzilla.xamarin.com/show_bug.cgi?id=964