docs: usb: remove some broken references

It seems that some files were removed from USB documentation.

Update the links accordingly.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/00008303fde6b4e06d027d3b76ae7032614a7030.1580193653.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/usb/index.rst

@@ -22,11 +22,9 @@ USB support
     misc_usbsevseg
     mtouchusb
     ohci
-    rio
     usbip_protocol
     usbmon
     usb-serial
-    wusb-design-overview
 
     usb-help
     text_files

Documentation/usb/text_files.rst

@@ -16,12 +16,6 @@ USB devfs drop permissions source
 .. literalinclude:: usbdevfs-drop-permissions.c
     :language: c
 
-WUSB command line script to manipulate auth credentials
--------------------------------------------------------
-
-.. literalinclude:: wusb-cbaf
-   :language: shell
-
 Credits
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