To be done after 1.16-stable is branched.

Scan the codebase for deprecations marked with a 1.17 deadline and remove them (ack deprecation.*1.17). All deprecations with a 1.17 deadline have been shipped (at least) in 1.15 and 1.16.

Marked for a 1.16.0 release so it's done at the time of 1.16-stable branching, but please move to 1.17.0 when done.