MAINT-1891: FIX Updating the LLPipeline::clearRebuildGroups() to preserve HUD...

MAINT-1891: FIX Updating the LLPipeline::clearRebuildGroups() to preserve HUD groups.  clearRebuildGroups() is called during the teleport process.  In the previous state, HUD objects were being incorrectly removed from the rebuild groups during the teleport process.  This change will preserve HUD groups and should resolve this issue.