doc: Drop documentation of fswatch1

fsinfo supersedes it, and we don't want new code to use this channel.

Nothing in the whole cockpit world uses this any more, other than the
cockpit.file.watch() fallback when fsinfo isn't available.

doc/protocol.md

@@ -1016,37 +1016,6 @@ The life-cycle of the channel works like this:
 
  - The channel will stay open until the client closes it.
 
-Payload: fswatch1
------------------
-
-You will get a stream of change notifications for a file or a
-directory.
-
-The following options can be specified in the "open" control message:
-
- * "path": The path name to watch.  This should be an absolute path to
-   a file or directory.
-
-Each message on the stream will be a JSON object with the following
-fields:
-
- * "event": A string describing the kind of change.  One of "changed",
-   "deleted", "created", "attribute-changed", "moved", or "done-hint".
-
- * "path": The absolute path name of the file that has changed.
-
- * "other": The absolute path name of the other file in case of a "moved"
-   event.
-
- * "type": If the event was created this contains the type of the new file.
-   Will be one of: file, directory, link, special or unknown.
-
-In case of an error, the channel will be closed.  In addition to the
-usual "problem" field, the "close" control message sent by the server
-might have the following additional fields:
-
- * "message": A string in the current locale describing the error.
-
 Payload: fsread1
 ----------------