SI-7603 Fix thread safety of FlagTranslation

This is outside of the Global cake, so we can't assume single
threaded access. A var was introduced in af3daf6fdb that can lead
to incorrect flag interpretation. The reported bug was triggered
in a multi-project SBT build. Java Annotations read from classfiles
were occasionally conferred the TRAIT|INFERFACE|ABSTRACT flag set,
leading to "@Foo is abstract, cannot be instatiated" later down the
line.
SI-7603 Remove diagnostic code for annotation error

We've fingered the true culprit in the previous commit.
Merge pull request #2842 from retronym/ticket/7603-2

SI-7603 Fix thread safety of FlagTranslation