tcp_hpts: use tcp_pace.cts_last_ran for last ran table

Remove the global cts_last_ran and use already existing unused field of
struct tcp_hptsi, which seems originally planned to hold this table. This
makes it consistent with other malloc-ed tables, like main array of HPTS
entities and CPU groups.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D43091