Deprecation for ember data mixin for older ED versions

[mydea]

After #308, we get a lot of deprecation warnings in our app:

DEPRECATION: FetchAdapter is deprecated, it is no longer required for ember-data>=3.9.2 [deprecation id: deprecate-fetch-ember-data-support]

We cannot easily update ED (for other reasons), so we are currently using 3.4.x.

The deprecation warning incorrectly suggests that we should stop using the FetchAdapter. I guess the deprecation should only be shown for users with ember-data >= 3.10?

[Turbo87]

I'm seeing the same issue on https://github.com/rust-lang/crates.io

/cc @xg-wang

[xg-wang]

@stefanpenner #447 will be contradicting the request here to only log deprecation for newer ember-data versions. If we're going to drop the mixin, answer to this issue is to manually silence the deprecation until ember-data upgrade is finished.

@mydea @Turbo87 what do you think?

[mydea]

I guess that's OK, maybe not ideal but workable!