Cleanup: remove BLENDER_USER_AUTOSAVE

This was only used on WIN32 when the temporary directory didn't exist.
When the check was added [0] this made some sense because it relied on
`U.tempdir` existing, since then additional checks have been added to
ensure a temporary directory can be used. Further, this fall-back
location isn't documented in the user manual.

[0]: 615db01b01

scripts/modules/bpy/utils/__init__.py

@@ -739,7 +739,7 @@ def user_resource(resource_type, *, path="", create=False):
     """
     Return a user resource path (normally from the users home directory).
 
-    :arg type: Resource type in ['DATAFILES', 'CONFIG', 'SCRIPTS', 'AUTOSAVE'].
+    :arg type: Resource type in ['DATAFILES', 'CONFIG', 'SCRIPTS'].
     :type type: string
     :arg path: Optional subdirectory.
     :type path: string