Avoid warning about multiple graphviz directives

[2bndy5]

This PR fixes #803 (a priority bug in v4.33.0).

• remove duplicated config options inherent to sphinx.ext.graphviz. This was causing a warning in sphinx build logs.
• remove attempt to register graphviz directive. This is needed to allow users to arbitrarily add breathe and sphinx.ext.graphviz to the conf.py extensions list in whatever order.

As stated already in the docs (added in #757), using the dot tool requires that sphinx.ext.graphviz be added to the extensions list in conf.py. Appropriate warnings will get triggered when breathe uses the graphviz directive without adding sphinx.ext.graphviz to the extensions list.

Just to reiterate what is said in the docs:

• sphinx.ext.graphviz is required for the \dot and \dotfile cmds.
• :allow-dot-graphs: (an option for doxygenclass, doxygenstruct, doxygenfile, doxygenindex
autodoxygenfile, and autodoxygenindex directives) is required for the doxygen-generated graphs (like include/inheritance/collaboration graphs), since sphinx.ext.graphviz is required to use the graphviz directive that will render the doxygen-generated graphs. Without specifying :allow-dot-graphs:, the doxygen generated graphs will be ignored (even if sphinx.ext.graphviz is added to the list of used extensions in conf.py).

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@gmarull did the right thing by commenting on #757. Otherwise, I would not have been alerted to #803.

Please tag me ( @2bndy5 ) in the future if there are any other problems that arise from the feature introduced by #757. 🤞🏼

[2bndy5]

I've been testing these changes on the same sample project used in #757, but with the -W flag asserted.

[2bndy5]

This seems to be high priority given the amount of references in #803

I can't think of any other ways/scenarios to test these changes on. Everything looks good and ready for review. Again, I'm very sorry I didn't catch this before v4.33.0 release.