TCP BBR: remove code which is not needed

rc_bbr_substate is a 3-bit unsigned int, so it can't be larger than
or equal to 8. The wrap around already happens.
No functional change intended.

Reviewed by: rrs
CID: 1523795
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48320