Fix an issue where UsdSkelImagingSkeletonAdapter::_RemovePrim() failed

to remove skeletons that did not have any bindings to skinned prims.

This could lead to crashes later on from accessing expired prims.

UsdSkelImagingSkeletonAdapter::_RemovePrim() checks _skelBindingMap to
determine whether the provided path is the skeleton prim. This was
inconsistent with UsdSkelImagingSkeletonAdapter::ProcessPrimResync(),
which checks _skelDataCache via _GetSkelData(). If there aren't any
bindings, the skeleton is in _skelDataCache but not _skelBindingMap.

Fixes: PixarAnimationStudios#1228, PixarAnimationStudios#1248

pxr/usdImaging/usdSkelImaging/skeletonAdapter.cpp

@@ -984,8 +984,7 @@ UsdSkelImagingSkeletonAdapter::_RemovePrim(const SdfPath& cachePath,
 
     // Alternative way of finding whether this is a callback for the skeleton/
     // bone mesh.
-    bool isSkelPath = _skelBindingMap.find(cachePath) != _skelBindingMap.end();
-    if (isSkelPath) {
+    if (_GetSkelData(cachePath)) {
 
         TF_DEBUG(USDIMAGING_CHANGES).Msg(
                 "[SkeletonAdapter::_RemovePrim] Remove skeleton"