tcp: rearrange enum and remove unused variable

Rearrange the enum tt_which such that TT_REXMIT is 0. This allows
an extension of the BBLog event RTO in a backwards compatible way.
Remove tcptimers, which was only used in trpt, a utility removed
from the source tree recently.

Reviewed by: glebius@, guest-ccui@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D38547