Remove AssumeAsgTagPropagation

[brycahta]

Issue #, if available: #616

Description of changes:

• Removes AssumeAsgTagPropagation flag, config, logic, and tests because instances should ways be tagged
• Leverage ASG APIs iff checkASGTagBeforeDraining: true (default)

Context:

• The original PR introducing AssumeAsgTagPropagation also mentions this is probably not needed:

I left in the code which, in cases where an instance does not have an aws:autoscaling:groupName tag, assumes that the instance might nonetheless belong to an ASG and calls the autoscaling API to make sure. I'm not sure these code paths need to exist. Is that aws:autoscaling:groupName tag a Sufficiently Official AWS Feature that we can get rid of that logic and just assume the tag will always be there?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.