Add method for removing the current stage

scripts/Windows/Scripts/Context.ps1

@@ -58,6 +58,10 @@ class Context {
         return $this.Get($this.StagePropertyName);
     }
 
+    [void] RemoveStage() {
+        $this.Remove($this.StagePropertyName);
+    }
+
     [string] ArchivePath($name) {
         return Join-Path $this.BackupRoot() "$name.7z";
     }