hioOiio: remove use of OIIO_NAMESPACE_USING macro

This macro was removed in OpenImageIO 3.1, but it previously expanded
simply to "using namespace OIIO;" and had been that way since at least
OpenImageIO v2.0.14, so we can just use that directly instead.

Author: mattyjams

pxr/imaging/plugin/hioOiio/metadata.cpp

@@ -18,7 +18,7 @@ PXR_NAMESPACE_OPEN_SCOPE
 class TfToken;
 class VtValue;
 
-OIIO_NAMESPACE_USING
+using namespace OIIO;
 
 bool
 HioOIIO_ExtractCustomMetadata(

pxr/imaging/plugin/hioOiio/oiioImage.cpp

@@ -33,7 +33,7 @@ ARCH_PRAGMA_POP
 
 PXR_NAMESPACE_OPEN_SCOPE
 
-OIIO_NAMESPACE_USING
+using namespace OIIO;
 
 // _ioProxySupportedExtensions is a list of hardcoded file extensions that
 // support ioProxy. Although OIIO has an api call for checking whether or