Revert "EtoEnvironment.Platform: Add iOS and Android detection. Fixes picoe/Eto#182."

This reverts commit 4383e7e0b27c97828c7a5c5b65ba95fa9f86cea0.
Revert "Fix Generator.Detect to use System.Environment.OSVersion.Platform."

This reverts commit bff2dcdd1b3f611977758a8f28f8da3c7510e62f.
Add Generator.IsAndroid