[BUG]: Duplicate FigureWidget import code in graph_objects and graph_objs #5849

Description

The FigureWidget import code is duplicated in graph_objects and graph_objs.

Steps to reproduce

• Open codegen/__init__.py and find optional_figure_widget_import.
• Note that the codegen passes the string to write_init_py for graph_objs and to build_from_imports_py for graph_objects.
• Open plotly/graph_objs/__init__.py and plotly/graph_objects/__init__.py.
• Note the identical FigureWidget blocks at the end of each file.

Notes

• The codegen writes the same optional FigureWidget import block into two generated files: plotly/graph_objs/__init__.py and plotly/graph_objects/__init__.py
• graph_objects already re-exports every other class from graph_objs, including Figure. FigureWidget can follow the same path, because graph_objs handles the optional ipywidgets import and the missing_* fallback.
• Any fix should not change behavior. This is just about removing duplicate code.
• Related:
  • remove duplicate FigureWidget code in graph_objects #3779 (closed, edited the generated file by hand)
  • re-write test for missing FigureWidget #5238 (test for the missing FigureWidget fallback)