Fix unused variable warning in tcp_stacks's bbr.c

With clang 15, the following -Werror warning is produced:

sys/netinet/tcp_stacks/bbr.c:11925:11: error: variable 'rtr_cnt' set but not used [-Werror,-Wunused-but-set-variable]

uint32_t rtr_cnt = 0;
         ^

The 'rtr_cnt' variable was in bbr.c when it was first added, but it
appears to have been a debugging aid that has never been used, so remove
it.

MFC after: 3 days