ruby: cleanup object_destroy()

ruby: cleanup object_destroy()

It was assumed the destructor of notmuch_rb_database_type did return a
notmuch_status_t because that's what notmuch_database_close returns, and
that value was checked by notmuch_rb_database_close in order to decide
if to raise an exception.

It turns out notmuch_database_destroy was called instead, so nothing was
returned (void).

All the destroy functions are void, and that's what we want.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>