*-user: remove the guest_user_syscall tracepoints

*-user: remove the guest_user_syscall tracepoints

This is pure duplication now. Both bsd-user and linux-user have
builtin strace support and we can also track syscalls via the plugins
system.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20230526165401.574474-2-alex.bennee@linaro.org
Message-Id: <20230524133952.3971948-2-alex.bennee@linaro.org>
[Remove unused variable in do_freebsd_syscall() reported by Richard
Henderson.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>