Remove sort(...;order=...) deprecation.

* Per @StefanKarpinski, the order keyword was deprecated with the
  expectation that functions would be specialized soon.  As this does
  not seem to be the case, the keyword is still useful when
  performance is important.
Merge pull request #7528 from JuliaLang/kms/remove_order_deprecation

Remove sort(...;order=...) deprecation.