AEP-0203: remove IDENTIFIER annotation.

[toumorokoshi]

          I really don't like this annotation - it's very confusing, and it basically means that the behavior of the field isn't always an input. 

I think there's actually a quick fix to this one - we make the field within the body of the resource always OUTPUT_ONLY, and add a field path into the Update method. Could we raise this in the next AEP meeting?

Originally posted by @toumorokoshi in #170 (comment)