Deduplicate custom_directives.py

[Eric-Arellano]

Follow up to #262, which removes the Sphinx Gallery directives. Confirm we are using them in every repository and delete if not. If enough people are using them, migrate it to qiskit_sphinx_theme.

❯ fd custom_directives.py
qiskit-finance/docs/custom_directives.py
qiskit-machine-learning/docs/custom_directives.py
qiskit-metapackage/docs/custom_directives.py
qiskit-nature/docs/custom_directives.py
qiskit-optimization/docs/custom_directives.py

[Eric-Arellano]

Turns out that the only directive people are now using is customcalloutitem customcarditem. I have finished opening PRs to remove the 3 other directives from each project.

So, the next steps are:

1. Confirm we want to keep customcalloutitem and customcarditem
2. Move them into qiskit_sphinx_theme.