Revert "Add support for NULLS FIRST/LAST in ORDER BY"

Despite being an ISO/ANSI SQL feature, it's apparently not supported by
enough databses to have by default.

This reverts commits 2266ca5c0bf34c24ba7fbf6448ad1c34a082f240 and
b137b0744a3aaea3df1ba497345378e9d3f8da40.