Remove Preconditions class

[kimchy]

We don't need our own anymore, since we removed the Elasticsearch illegal argument exception

[javanna]

LGTM

[s1monw]

can we use Objects.requireNonNull(T obj, String message) instead?

[kimchy]

@s1monw I first wanted to remove the Preconditions class first that we don't need anymore

[s1monw]

ok not sure if it's worth 2 PRs ?

[kimchy]

its a bigger change, since there are other places that use guava preconditions checkNonNull, also, we can't replace most of the ones here, which are for illegal argument

[s1monw]

ok fair enough!