sched_shim: Drop the no-ifunc case

Now all architectures support kernel ifunc resolvers. Therefore, the
alternate implementation can be removed.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55114

(cherry picked from commit 0d3652f67d246348e2c017205c6782caf4484449)