Remove obsoletion of Member methods

I've removed the obsolete methods from the member class because many of
the suggested alternatives don't work. and some require a lot of
configuration to get working.

Also, the member methods return Members - so they are not obsolete if
you want a reference to a member, you can choose to use System
membership if you want, but for custom properties and groups you need to
fallback to the Umbraco API, so these shouldn't be obsolete! :)