Fix unused variable warning in xen's blkback.c

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

sys/dev/xen/blkback/blkback.c:1561:12: error: variable 'req_seg_idx' set but not used [-Werror,-Wunused-but-set-variable]
                u_int                    req_seg_idx;
                                         ^

The 'req_seg_idx' variable was used in the for loop later in the
xbb_dispatch_io() function, but refactoring in 112cacaee408 got rid of
it. Remove the variable since it no longer serves any purpose.

MFC after: 3 days