tdf#143148: Use pragma once instead of include guards

Change-Id: I2fc2ca48c51b308379eebde77a40075e044c8587
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/201968
Tested-by: Jenkins
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
Reviewed-by: Dmitriy Usov <du@thesec.net>

registry/source/keyimpl.hxx

@@ -17,8 +17,7 @@
  *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
  */
 
-#ifndef INCLUDED_REGISTRY_SOURCE_KEYIMPL_HXX
-#define INCLUDED_REGISTRY_SOURCE_KEYIMPL_HXX
+#pragma once
 
 #include <sal/config.h>
 
@@ -134,7 +133,4 @@ private:
     ORegistry*               m_pRegistry;
 };
 
-#endif
-
-
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */