iavf: remove unused debug function iavf_debug_d

There is no caller of function iavf_debug_d() in tree since
commit 75051ce4 ("iavf: Fix up debug print macro"),
so it can be removed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>