ixgbe: don't check NULL for debugfs_remove_recursive

The debugfs_remove_recursive function is NULL-safe, so we don't need to
check here ourselves.

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>