(PUP-2798) Remove warnings for reserved words in 3x regular.

We are issuing too many deprecation warnings for people to keep up.
The migration paths is to use the future parser/evaluator to "future
proof" manifests.

This only changes the warnings for 3x regular. And is basically a
reversal of part of PUP-1027 where these warnings were added).
Merge pull request #2781 from hlindberg/PUP-2798_Stop-reserved-word-warning-3x

(PUP-2798) Remove warnings for reserved words in 3x regular.