api: remove unused RootFS.BaseLayer

This field was used when Windows did not yet support regular images, and required
the base-image to pre-exist on the Windows machine (as those layers were not yet
allowed to be distributed).

Commit f342b27145d8f5af27cd5de1501551af275e899b (docker 1.13.0, API v1.25) removed
usage of the field.

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

api/types/types.go

@@ -19,9 +19,8 @@ import (
 
 // RootFS returns Image's RootFS description including the layer IDs.
 type RootFS struct {
-	Type      string
-	Layers    []string `json:",omitempty"`
-	BaseLayer string   `json:",omitempty"`
+	Type   string   `json:",omitempty"`
+	Layers []string `json:",omitempty"`
 }
 
 // ImageInspect contains response of Engine API: