ixgbe: remove IXGBE_FLAG_MSI(X)_CAPABLE flags

They were not used, and we don't need them, so we shouldn't bother with
keeping values in the flags field that could be misleading.

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>