TCP BBR: remove dead code

bw is unsigned and not zero. So it cannot be smaller than 1.
No functional change intended.

Reviewed by: rrs, cc
CID: 1523791
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48323

(cherry picked from commit c28fefe1dc44b69743dd18d038440da38a2867a7)