Drop the "created from" line from files generated by makesyscalls.sh.

This information is less useful when the generated files are included in
source control along with the source. If needed it can be reconstructed
from the $FreeBSD$ tag in the generated file. Removing this information
from the generated output permits committing the generated files along
with the change to the system call master list without having inconsistent
metadata in the generated files.

Reviewed by: emaste, kib
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D9497