[multi-sources] closes #1918120: drop behaviour leading to wrong result as demonstrated by the ticket The formerly implemented behaviour doesn't seems to be useful, nor to have a chance to be correct. At least no test case demonstrate the opposite, so I've dropped it.

doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc