mc: patch geompropvalue nodes instead of relying on default values

This behaviour was recently changed, but it causes the primvar
detection heuristics (e.g. texcoord names) to be ignored. It will
be restored once gatling properly supports arbitrary primvar reading.

src/mc/impl/MtlxDocumentPatcher.cpp

@@ -475,10 +475,7 @@ namespace gtl
     }
 #endif
 
-    // MaterialX PR #1417 introduced geompropvalue support in 1.38.9+ although it's not advertised in the changelog.
-#if (MATERIALX_MAJOR_VERSION == 1 && MATERIALX_MINOR_VERSION == 38 && MATERIALX_BUILD_VERSION < 9)
     _PatchGeomprops(document);
-#endif
 
     _PatchNodeNames(document);