Remove TfIsFile check in UsdSchemaRegistry

This check was unnecessary and interferes with SdfLayer's
own file existence checks. In particular, it would disallow
the use of symlinks to a generatedSchema.usda file.

Fixes #763

(Internal change: 1948614)

pxr/usd/lib/usd/schemaRegistry.cpp

@@ -124,7 +124,7 @@ _GetGeneratedSchema(const PlugPluginPtr &plugin)
     // Look for generatedSchema in Resources.
     const string fname = TfStringCatPaths(plugin->GetResourcePath(),
                                           "generatedSchema.usda");
-    return TfIsFile(fname) ? SdfLayer::OpenAsAnonymous(fname) : TfNullPtr;
+    return SdfLayer::OpenAsAnonymous(fname);
 }
 
 void