top(1): remove last vestiges of "last pid" support

top(1): remove last vestiges of "last pid" support

millert@ removed most of the "last pid" support from top(1) in 1997.
See, e.g. top/machine.c,v1.7:

http://cvsweb.openbsd.org/src/usr.bin/top/machine.c?rev=1.7&content-type=text/x-cvsweb-markup

Let's remove the rest of it:

- Eliminate system_info.last_pid.

- Remove mpid parameter and "last pid" printing code from i_loadave().

Link: https://marc.info/?l=openbsd-tech&m=166277253606823&w=2
ok millert@