inpcb: don't return INP_DROPPED entries from pcb lookups

The in_pcbdrop() KPI, which is used solely by TCP, allows to remove a
pcb from hash list and mark it as dropped. The comment suggests that
such pcb won't be returned by lookups. Indeed, every call to
in_pcblookup*() is accompanied by a check for INP_DROPPED. Do what
comment suggests: never return such pcbs and remove unnecessary checks.

Reviewed by: tuexen
Differential revision: https://reviews.freebsd.org/D37061