build: remove references to unused optional dependencies
Any uses of libaccounts-glib and libsignon-glib were removed in 3.31.1 / 66121edf
I'm opening this MR mostly because nix was still providing libaccounts-glib as a dependency to evolution-data-server, and this should hopefully avoid any further confusion (though I think we're the only ones).
Edited by Maxine Aubrey