Remove regex_flavor GUC, so that regular expressions are always "advanced"

Remove regex_flavor GUC, so that regular expressions are always "advanced"
style by default.  Per discussion, there seems to be hardly anything that
really relies on being able to change the regex flavor, so the ability to
select it via embedded options ought to be enough for any stragglers.
Also, if we didn't remove the GUC, we'd really be morally obligated to
mark the regex functions non-immutable, which'd possibly create performance
issues.