Remove unused extension from sphinx config.
Remove unused extension from sphinx config.

We don't use breathe any more (because we don't use doxygen to extract
information from the code--everyone seems to use ctags to jump round
instead!) so we shouldn't force people to have it in order to compile
the documentation.

hande-qmc/hande,hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande,hande-qmc/hande,hande-qmc/hande