cpu_set_upcall / linux_clone_thread: Remove calls to cpu_thread_clean

This is intended to clean state of a thread at the end of its
lifecycle during wait(), not the beginning of its life cycle.

Reviewed by: kib
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D48023

(cherry picked from commit 0e742cc77454d0872ac2f6e7fd755b6d0322b711)