chore: remove support for ecr spec fallbacks for jumpstart models

[evakravi]

Issue #, if available:

Description of changes:
JumpStart metadata contains a legacy field called hosting_ecr_specs (and training_ecr_specs) which looks like this:

"hosting_ecr_specs": {
    "framework": "pytorch",
    "framework_version": "1.10.0",
    "py_version": "py38"
}

Using this metadata to resolve image uris for JumpStart models can cause invalid image uris to return, as we no longer rely on these fields for image uris.

Testing done:
Unit + integ tests pass

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[JGuinegagne]

thanks - I would vote for a warning but your call :)

technically, this is still backward incompatible, but the risk of breaking any customers is really low (it would require cx to have written code than calls the retrieve method w/ both a model_id and a framework, container_version and/or py_version which is unlikely.

[evakravi]

i was thinking of warning but thought it might spam the logs.

i don't see how we can make this not backwards. we can no longer rely on ecr specs for raising exceptions.

[JGuinegagne]

nit: that's not a mock, is it?