TCP BBR: remove code which is never executed
USEC_2_TICKS() returns at least 1.
Reviewed by: rrs
CID: 1523775
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D4827
(cherry picked from commit 1781324db2bc22a971a33c5a063036b2775055c6)