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

[glebius]

Summary

The HPTS input queue is in reality used only for "delayed drops".
When a TCP stack decides to drop a connection on the input path
it can't do that due to locking protocol between main tcp_input()
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.

[glebius]

Rebased on newer main.

[glebius]

Committed f971e791391d9f7ff6bfd7d7e0bed93267216329