Use warnings.pm to calculate warnings tree

This removes the hard coded warnings tree information, and instead uses the %Bits variable from warnings.pm to check what warnings categories belong to each other.  This ensures that the categories are up to date with the perl version installed, and removes the need for Tree::DAG_Node as a prerequisite.

Although %Bits isn't a documented part of the warnings API, it has existed with the same structure since lexical warnings were introduced.  This patch also doesn't depend any specific values in %Bits, so it should be safe across perl versions.

This also remove removes the $Test::Warn::Categorization:tree variable.  Although it was documented, it was discouraged and was never used on CPAN.