Completely drop unresolved generic references

Generic reference get a special handling, due to how Emacs handles
references to non-existing symbols.  Emacs does simply not convert them
to references at all, so instead of reporting an unresolved symbol
reference to Sphinx, we drop such references entirely and replace them
with their contents, minus any xref classes.

Note that non-generic references, e.g. function or variable references,
are still never dropped, and always reported as unresolved, if the
target doesn't exist.