Remove original handling on ol.Feature

This is another attempt to bring master closer to the
vector-api branch. In anticipation of the ability to keep track
of modifications on ol.Object through a beforechange event
(d7e4be0), we will be able to manage originals on the
application level or in a separate component outside of
ol.Feature.
Merge pull request #1347 from ahocevar/no-original

Remove original handling on ol.Feature