Fix figure legend when drawing stackplots

[dstansby]

PR summary

Fixes #30158 - as suggested in #30158 (comment), two calls were trying to set the figure, and I think one of them doesn't need to.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[QuLogic]

Yes, I believe this is correct; HandlerBase.update_prop will set the correct figure (via Legend._set_artist_props), HandlerPolyCollection._update_prop exists to do the class-specific work of pulling the properties from the first artist in a collection and doesn't need to set the figure.

This could do with a test though.

[rcomer]

Seems right to me too. The legend and the artists it contains (e.g. legend_handle here) are on the root Figure, whereas orig_handle is on the SubFigure, so they should not match.

[tacaswell]

@meeseeksmachine please backport to 3.10.x

[rcomer]

@meeseeksdev backport to v3.10.x