Fix unused variable warning in xen's blkfront.c

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

sys/dev/xen/blkfront/blkfront.c:602:6: error: variable 'sbp' set but not used [-Werror,-Wunused-but-set-variable]
        int sbp;
            ^

The 'sbp' variable was used in the for loop later in the xb_dump()
function, but refactoring in e4808c4b2dba got rid of it. Remove the
variable since it no longer serves any purpose.

MFC after: 3 days

(cherry picked from commit e635220e1a330560ee547069054d81c4cf0b7839)