tcp_hpts: rename input queue to drop queue and trim dead code

The HPTS input queue is in reality used only for "delayed drops".
When a TCP stack decides to drop a connection on the output path
it can't do that due to locking protocol between main tcp_output()
and stacks. So, rack/bbr utilize HPTS to drop the connection in
a different context.

In the past the queue could also process input packets in context
of HPTS thread, but now no stack uses this, so remove this
functionality.

Reviewed by: rrs
Differential revision: https://reviews.freebsd.org/D33025