[media] USB: Deletion of unnecessary checks before three function calls

GIT_AUTHOR_DATE=1416486805
The functions pvr2_hdw_destroy(), rc_unregister_device() and vfree() perform
also input parameter validation. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>