Revert "virttest.qemu_vm: Add freq option to usb device"

This reverts commit fadf99e6c0b1bdd86315040b3d3a35a804a550e8.

The freq option was removed in commit:
https://github.com/autotest/autotest/commit/93acc8819c8d912f2f3bf6f4d5d66bc2c99181a4
Thus revert this commit from virt-test.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>