xen-netback: remove unused variables pending_idx and index

xen-netback: remove unused variables pending_idx and index

building with gcc and W=1 reports
drivers/net/xen-netback/netback.c:886:21: error: variable
  ‘pending_idx’ set but not used [-Werror=unused-but-set-variable]
  886 |                 u16 pending_idx;
      |                     ^~~~~~~~~~~

pending_idx is not used so remove it.  Since index was only
used to set pending_idx, remove index as well.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230226163429.2351600-1-trix@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>