ixgbe: remove unused enum latency_range

This enum is no longer needed after
commit: b4ded832 ("ixgbe: Update adaptive ITR algorithm")

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>