sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Support for Alpine Serializer/Deserializer.

The exported functions will be used by
Alpine Ethernet driver.

Obtained from: Semihalf
Submitted by: Michal Stanek <mst@semihalf.com>
Sponsored by: Annapurna Labs
Reviewed by: wma
Differential Revision: https://reviews.freebsd.org/D7763