Eliminates $?GRAMMAR

Which does not seem to be either in rakudo or roast or, for that
matter, work. Closes #2547

Author: JJ

doc/Language/variables.pod6

@@ -1147,7 +1147,6 @@ The following compile time variables allow for a deeper introspection:
     $?MODULE         Which module am I in?
     $?CLASS          Which class am I in? (as variable)
     $?ROLE           Which role am I in? (as variable)
-    $?GRAMMAR        Which grammar am I in?
     $?TABSTOP        How many spaces is a tab in a heredoc or virtual margin?
     $?NL             What a vertical newline "\n" means: LF, CR or CRLF
     $?DISTRIBUTION   The Distribution of the current compilation unit.