libebook: Don’t warn on calling unimplemented methods

If a warning is needed, it should be in the method itself. Otherwise we
can be outputting spurious warnings for methods which don’t need
implementing.

This fixes the libfolks EDS tests.

https://bugzilla.gnome.org/show_bug.cgi?id=731981
libecal: Don’t warn on calling unimplemented methods

See commit d1f237c2e455dda7c91ca79a6f643ef1a613bb61 for rationale.

https://bugzilla.gnome.org/show_bug.cgi?id=731981