diagnostics: no longer pretty-print rules in error messages, carets suffice

diagnostics: no longer pretty-print rules in error messages, carets suffice

* src/gram.c (grammar_rules_useless_report): Let -fcaret handle the
pretty-printing of the guilty rules.
(rule_print): Inline in its only use.
* tests/conflicts.at, tests/existing.at, tests/reduce.at,
* tests/regression.at: Adjust.
* NEWS: Document.