remove references in documentation to defunct pxrUsdTranslators plugin

Besides removing references to the now non-existent pxrUsdTranslators plugin,
this also adjusts some of the language in the Pixar plugin documentation to
suggest favoring the mayaUsd library and the mayaUsdPlugin over pxrUsd.

plugin/pxr/maya/lib/usdMaya/overview.dox

@@ -7,10 +7,8 @@
 USD support in Maya is provided through several libraries and plugins:
 - \subpage usd_maya_lib_usdMaya_page_front
   This library provides reusable logic for using USD in Maya.
-- \subpage usd_maya_plugin_pxrUsdTranslators_page_front
-  This plugin supports the built-in Maya nodes.
 - \subpage usd_maya_plugin_pxrUsd_page_front
-  This is the main plugin that you load for USD functionality in Maya.
+  This is the main plugin that you load for Pixar's USD functionality in Maya.
 - \subpage usd_maya_plugin_pxrUsdPreviewSurface_page_front
   This is a Maya surface shader plugin node that mimics the UsdPreviewSurface shader in USD.
 
@@ -31,9 +29,7 @@ proxy shapes, and related Maya nodes.
 ## Translation Plugins
 
 Plugins can provide new translation functions for USD types that are not built
-into the core UsdMaya library. We ship plugins for the major Maya built-in node
-types in the \ref usd_maya_plugin_pxrUsdTranslators_page_front
-"pxrUsdTranslators Maya plugin".
+into the core UsdMaya library.
 
 When implementing new plugins, you must make sure to generate plugInfo.json
 files, and that they are recognized by the Plug system. More details for

plugin/pxr/maya/plugin/pxrUsd/overview.dox

@@ -1,20 +1,19 @@
 /*!
-\page usd_maya_plugin_pxrUsd_page_front PxrUsd : Main USD plugin for Maya
+\page usd_maya_plugin_pxrUsd_page_front PxrUsd : Pixar's USD plugin for Maya
 \if ( PIXAR_MFB_BUILD )
-\mainpage PxrUsd : Main USD plugin for Maya
+\mainpage PxrUsd : Pixar's USD plugin for Maya
 \publicLib
 \endif
 
 ## Overview
 
-This is the main plugin that registers USD functionality in Maya, including
-import and export commands, as well as USD nodes such as the reference assembly
-and proxy shape.
+This is the plugin that registers Pixar's USD functionality in Maya, including
+import and export commands, as well as USD nodes such as Pixar's USD reference
+assembly and the Pixar-specific USD proxy shape.
 
-The core reusable logic is implemented in the
-\ref usd_maya_lib_usdMaya_page_front "UsdMaya library". Translation
-functionality for built-in Maya types is automatically loaded from the
-\ref usd_maya_plugin_pxrUsdTranslators_page_front "pxrUsdTranslators plugin"
-when needed.
+The majority of the functionality has been absorbed into the core mayaUsd
+library, and the "mayaUsdPlugin" should be favored over "pxrUsd". What remains
+in the Pixar plugin is implemented in the
+\ref usd_maya_lib_usdMaya_page_front "UsdMaya library".
 
 */