Remove enum valueOf methods from nullability_warnings.txt.

The other change in this topic makes metalava exempt these methods
from nullability validation, since they don't exist in the source
code. This change updates the list of known warnings accordingly (it
is now empty).

Bug: 73448108
Test: make core-current-stubs-nullability-validation-check-nullability-warnings
Change-Id: Ib943c121723355133056e53b9c3ce7dd92ec7617
1 file changed