ixgbe: Remove an obsolete comment about ITR

The InterruptThrottleRate has been removed from ixgbe. Then Update
the comment.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>