Parser: remove the dead cover parenthesized production

Parser: remove the dead cover parenthesized production

Previously, njs_parser_cover_parenthesized_expression(), its two
continuations and njs_parser_binding_identifier_pattern() formed a closed
island: every reference to them came from inside it, and the only thing
linking it to the parser was a "(void)" cast left with a TODO in 2020.  It
honours neither contract this series introduces.  Two of its states are
reject landing states that never inspected parser->ret, so with a ")" or a
"," in hand they would consume the token and pop the stack after a reject.
Its parenthesized expression opens no [+In] region.

The fix is to delete the island rather than repair it.  Arrow parameters are
not going to be completed through this production, and the binding pattern
entry points it dispatched to are stubs reachable on their own.