Remove the `_boost` field

[clintongormley]

The _boost field was deprecated in 1.0.0.RC1 in favour of using the function_score query with an ordinary field. It should be removed in 2.0.0. Any mapping which includes a _boost field should be updated automatically to remove it.

Relates to #8870

[reuben-sutton]

@clintongormley Is there a mechanism for automatically updating the mapping like this? I had a look, but couldn't figure it out.

[clintongormley]

@reuben-sutton there isn't, currently