gh-788: remove target validation again
gh-788: remove target validation again

After some more thinking we can't introduce a lot of validation.
Basically anything that would force us to do a read-before-write would
essentially defeat the point of the batch importer. For more details,
please see https://github.com/creativesoftwarefdn/weaviate/issues/788#issuecomment-474335506

weaviate/weaviate,weaviate/weaviate