kern: syscall_thread_enter() cannot fail

Attempting to handle the error gracefully can easily result in missing
SIGSYS, so this was made to always succeed in
39024a89146 ("syscalls: fix missing SIGSYS for several ENOSYS errors")
and returns the nosys entry on failure.

Drop the pretense of returning an error and clean up a few dead error
paths.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D57848