qvm-usb: remove scary warning about PV USB stability

USBIP (used for PV USB here) is considered stable by Linux maintainers,
so follow their judgement.

Fixes QubesOS/qubes-issues#531

qvm-tools/qvm-usb

@@ -27,8 +27,6 @@ from optparse import OptionParser
 import sys
 import os
 
-pvusb_enable_flagfile = '/var/lib/qubes/pvusb-enable.flag'
-
 def main():
     usage = "usage: %prog -l [options]\n"\
             "usage: %prog -a [options] <vm-name> <device-vm-name>:<device>\n"\
@@ -49,24 +47,6 @@ def main():
 
     (options, args) = parser.parse_args ()
 
-    if not os.path.exists(pvusb_enable_flagfile):
-        print >> sys.stderr, ""
-        print >> sys.stderr, "******* WARNING *** WARNING *** WARNING *** WARNING *******"
-        print >> sys.stderr, "***                                                     ***"
-        print >> sys.stderr, "*** PVUSB passthrough kernel support is still unstable. ***"
-        print >> sys.stderr, "***                It can CRASH your VMs.               ***"
-        print >> sys.stderr, "***                                                     ***"
-        print >> sys.stderr, "***********************************************************"
-        print >> sys.stderr, ""
-        print >> sys.stderr, "To use it, you need install kernel from \"unstable\" repository"
-        print >> sys.stderr, "If you still want to enable it, type capital YES"
-        print >> sys.stderr, ""
-        prompt = raw_input ("Do you want enable PV USB support? ")
-        if prompt == "YES":
-            open(pvusb_enable_flagfile, "w").close()
-        else:
-            exit(1)
-
     if hasattr(os, "geteuid") and os.geteuid() == 0:
         if not options.force_root:
             print >> sys.stderr, "*** Running this tool as root is strongly discouraged, this will lead you in permissions problems."