Until this change there were two places where we would free tcpcb -
tcp_discardcb() in case if all timers are drained and tcp_timer_discard()
otherwise.  They were pretty much copy-n-paste, except that in the
default case we would run tcp_hc_update().  Merge this into single
function tcp_freecb() and move new short version of tcp_timer_discard()
to tcp_timer.c and make it static.

Reviewed by:		rrs, hselasky
Differential revision:	https://reviews.freebsd.org/D32965