usb: core: remove flags variable in __usb_hcd_giveback_urb()

In commit ed194d13 ("usb: core: remove local_irq_save() around
->complete() handler") I removed the only user of the flags variable and
forgot to remove the variable, leading to warning because it is unused
now.
Remove the unused variable.

Fixes: ed194d13 ("usb: core: remove local_irq_save() around ->complete() handler")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>