Update IAMPolicy mixin to use <$service>.yaml file instead of explicit generator flag #612

As part of adding support generally for client library mixin method generations (draft AIP is at aip-dev/google.aip.dev#694), this update is to move the generator from reading the google.iam.v1.IAMPolicy mixin as an explicit generator flag to reading it from the <$service>.yaml file.

This change should not impact functionality in any way, as each API that had the explicit mixin flag should also have it listed in their <$service>.yaml file.

Ping me for the internal doc(s) if desired.