tcp : remove assignment without effect

rstreason is only relevant in the code paths with the label
'dropwithreset', but not in the one with the label 'drop'.
No functional change intended.

Reviewed by: Nick Banks, rrs, Peter Lei, imp
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D51814

(cherry picked from commit b6521ceaf2771fe6b3f4ae7e7eb390312e3a678a)