Remove redundant helper with inadequate comment. Improve types for builder.default_behavior_policy.
Remove redundant helper with inadequate comment. Improve types for builder.default_behavior_policy.

PiperOrigin-RevId: 397767886
Change-Id: Ib54fa9ed9864b9e3669dc57d1cdec1f1511e55c8

deepmind/acme,deepmind/acme