rc.subr: Remove misguided cpuset usage

When running an rc command, if the target rc script defines
<command>_cmd, e.g., start_cmd=..., then the run_rc_command() executes
that instead of $command. In general it's a shell function, and
"cpuset -l <n> <shell function>" doesn't work.

Moreover, it doesn't really make sense to run cpuset for anything other
than start_cmd.

Other optional isolation mechanisms (e.g., <name>_fib,
<name>_chroot) are only used when invoking $command directly as part of
the "start" command. Make <name>_cpuset consistent with everything else
by removing these extraneous cpuset invocations.

Reviewed by: 0mp
MFC after: 2 weeks
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53746

(cherry picked from commit 71f6592a01506899efd91306b6d8147f14a6b219)