Removal of operator_precedence_warning

I think it's time for $SUBJECT.  We added this GUC in 9.5, which
will be EOL by the time of our next major release, and it was never
meant as more than a transitional aid.  Moreover, it's been buggy
as heck (cf abb164655, 05104f693, 01e0cbc4f, 4cae471d1), and the
fact that some of those errors went undetected for years shows that
it's not really gotten much field usage.

Hence, I propose the attached.  Comments?

			regards, tom lane