hwpmc(4): remove paragraph about Pentium Pro handling

Support for these CPUs was removed in 2018 (e92a1350b50e).

Reviewed by: jkoshy, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40541

(cherry picked from commit 0a0c599ab9a578dd175e626144eacdbb4c5ac6bc)