tcp: cleanup

Remove the macros COPY_STAT and COPY_STAT_T, since they do not improve
the readability of the code. No functional change intended.
Thanks to glebius@ for suggesting the change.

Reviewed by: glebius, rrs
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D49670