Simplify information schema check constraint deparsing

Simplify information schema check constraint deparsing

The computation of the column
information_schema.check_constraints.check_clause used
pg_get_constraintdef() plus some string manipulation to get the check
clause back out.  This ended up with an extra pair of parentheses,
which is only an aesthetic problem, but also with suffixes like "NOT
VALID", which don't belong into that column.  We can fix both of these
problems and simplify the code by just using pg_get_expr() instead.

Discussion: https://www.postgresql.org/message-id/799b59ef-3330-f0d2-ee23-8cdfa1740987@eisentraut.org