Fixes to Axiom of Pairing in iset.mm
Fixes to Axiom of Pairing in iset.mm

- Remove zfpair and axpr; we don't have the version of Replacement
that they rely on, so just keep ax-pr as an axiom, rather than
saying it can be proved from Replacement.
- Cite Crosilla for ax-pr and adjust comments accordingly.
- Change section title from "Derive the Axiom of Pairing" to
"Axiom of Pairing"

metamath/set.mm,metamath/set.mm,metamath/set.mm