Remove 'nova-xvpvncproxy'

This legacy service is no longer used and was deprecated during the
Stein cycle [1]. It's time to say adios and remove them in their
entirety. This is pretty straightforward, with the sole exception of
schema for the 'remote-consoles' API, which has to continue supporting
requests for type 'xvpvnc' even if we can't fulfil those requests now.

[1] https://review.opendev.org/#/c/610076/

Part of blueprint remove-xvpvncproxy

Depends-On: https://review.opendev.org/695853
Change-Id: I2f7f2379d0cd54e4d0a91008ddb44858cfc5a4cf
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

api-ref/source/parameters.yaml

@@ -5284,7 +5284,7 @@ os-getVNCConsole:
   type: object
 os-getVNCConsole-type:
   description: |
-    The type of VNC console. The valid values are ``novnc`` and ``xvpvnc``.
+    The type of VNC console. The only valid value is ``novnc``.
   in: body
   required: true
   type: string
@@ -5833,7 +5833,7 @@ remote_console_protocol:
   type: string
 remote_console_type:
   description: |
-    The type of remote console. The valid values are ``novnc``, ``xvpvnc``,
+    The type of remote console. The valid values are ``novnc``,
     ``rdp-html5``, ``spice-html5``, ``serial``, and ``webmks``. The type
     ``webmks`` is added since Microversion ``2.8``.
   in: body