DOC: Remove last userdemo example

Nested gridspecs are explained in
`galleries/examples/subplots_axes_and_figures/gridspec_nested.py`, and
layout is more generally explained in
`galleries/users_explain/axes/arranging_axes.py` and
`galleries/users_explain/axes/constrainedlayout_guide.py`

Closes matplotlib#25800

Author: QuLogic