Remove 409 inProgressFault for deleteServer API

The compute v2 API allows you to request a delete of
a server while it's building since nova commit
c7d2f020f0fdf04b24bd21668e7a02796f1f5538, so this
change just updates the API docs.

Change-Id: I7444289d8e55d20e289894c526e5578a8d070c72
Closes-Bug: #1344925