Remove unused filter param from Swagger

Commit 745795ef2e added a `filter`
query-parameter to all "prune" endpoints, however the parameter was only used
when pruning images.

This patch removes the filter parameter from the volume-prune endpoint, given
that it is not used there, so there is no reason documenting it.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/swagger.yaml

@@ -6147,14 +6147,6 @@ paths:
       produces:
         - "application/json"
       operationId: "VolumePrune"
-      parameters:
-        - name: "filters"
-          in: "query"
-          description: |
-            Filters to process on the prune list, encoded as JSON (a `map[string][]string`).
-
-            Available filters:
-          type: "string"
       responses:
         200:
           description: "No error"