models/change: fix `id` property definition

Remove the flag `generated:true`, as it does not work together with a custom `getter.id` function.

This fixes a problem surfaced by https://github.com/strongloop/loopback-datasource-juggler/pull/323 and https://github.com/strongloop/loopback-datasource-juggler/pull/322.

Close https://github.com/strongloop/loopback-datasource-juggler/issues/326.

/to @ritch please review. Do you remember what was your original intention, why did you enable this flag in the first place?
/cc @raymondfeng