Drop "The permissions and context validity have been checked..." notes #501

A smattering of methods on MLGraphBuilder (build, buildSync, concat, constant, expand, and input) include this note at the start of their method steps:

The permissions and context validity have been checked by § 7.7.1 MLGraphBuilder constructor steps.

Presumably this applies to all methods of MLGraphBuilder, and it's unclear that it adds value. It's non-normative, so it's harmless, but... is it particularly meaningful for these methods -- and if so, why?