eina: remove the most costly boolean ever.
We did use this 'begin' boolean to make eina_hash always allocate only
once per item it push in the hash. This boolean was alone at the end of
a structure. It would have costed us 4bytes on 32bits system and 8bytes
on 64bits. Removing it make elemines consume 100KB less on 32bits system.
We may have a speed impact on hash insertion here, but I don't think we
do use eina_hash_add and friends in any hot path, at the moment. If that
was the case there would be some way to mitigate this, just not worth it
at the moment.