Remove use of usdaFileFormat from mjcf_file_format.

Previously this was just used for the USDA file format token, which we can confidently just construct ourselves.

Since [this PR in OpenUSD](https://github.com/PixarAnimationStudios/OpenUSD/commit/5bb9ad1fb2a034e1f9b48c7db936f296b5b78e25) the file lives in a different place now and public usage will be deprecated anyways.

PiperOrigin-RevId: 816801204
Change-Id: I42e92d2af52b0f9ff0a679ecb96f3d2dbf623531

src/experimental/usd/plugins/mjcf/mjcf_file_format.cc

@@ -39,7 +39,6 @@
 #include <pxr/usd/sdf/declareHandles.h>
 #include <pxr/usd/sdf/fileFormat.h>
 #include <pxr/usd/sdf/layer.h>
-#include <pxr/usd/usd/usdaFileFormat.h>
 
 PXR_NAMESPACE_OPEN_SCOPE
 
@@ -285,7 +284,7 @@ bool UsdMjcfFileFormat::Read(pxr::SdfLayer *layer,
 
 bool UsdMjcfFileFormat::WriteToString(const SdfLayer &layer, std::string *str,
                                       const std::string &comment) const {
-  return SdfFileFormat::FindById(pxr::UsdUsdaFileFormatTokens->Id)
+  return SdfFileFormat::FindById(pxr::TfToken("usda"))
       ->WriteToString(layer, str, comment);
 }