ShExC production rule 19 for shapeAtomNoRef is never used

[marcelotto]

The grammars from the https://github.com/shexSpec/grammar repo don't include this production at all, so I assume it can be removed here. If you confirm, I can create a PR.

[ericprud]

apologies for the delay -- We'll need to look at a couple implementations which pass all the syntax tests and make sure that i'm not simply missing the ref to shapeAtomNoRef. @hsolbrig , can you intelli-diff (i.e. look hard at) the delta between the 2.1 grammar and what's in the spec?

[marcelotto]

I will deliver an additional implementation (hopefully) soon, which will be in Elixir.

Stumbled across various issues with the current grammar in the specs to make the syntax tests pass. For some I've created PRs. Another problem I had, were with the tests using wildcard exclusions, for which the grammar is not up-to-date. But this wasn't even covered in the informal parts, so I was unsure about that.