tcp: retire rstreason

With the latest changes, this variable and parameter for
tcp_dropwithreset() is not needed anymore.
It also makes it harder to introduce the usage of multiple counters
for TCP again, which might open side channel attacks.
No funtional changes intended.

Reviewed by: rrs
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D51872