ixgbe: remove unused media type

This patch reverts the addition of the fiber_fixed type, which ended up
never being used. We don't have plans to support this type going
forward, and there is no reason to keep an unused type around polluting
the code.

Reverts: 4e8e1bca

 ("ixgbe: add new media type")
CC: Arun Sharma <asharma@fb.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>