net-mgmt/net-snmp: Fix freebsd15.h

It was copied from freebsd14.h without updating it.

Also remove some bogus copying in the port makefile. 5.9.4 provides
freebsdN.h for N up to 14, and we have a patch which adds freebsd15.h.
So, get rid of the copying too.

Approved by: zi
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45029