(PUP-3311) Remove deprecated provider methods
(PUP-3311) Remove provider model attribute

The provider model attribute was renamed to resource_type in 1d23013a
and a stub attr_reader was kept around since then. 7 years is a
reasonable time to maintain compatibility, but it's time to move on and
let the model attr_reader go out to pasture.
Merge pull request #3203 from adrienthebo/removal/master/pup-3311-provider-deprecated-methods

Removal/master/pup 3311 provider deprecated methods