Remove the warmer API

[jpountz]

With time, warmers have become less and less useful: we got doc values by default in 2.0 and disk-based norms in 2.1. Instead of fixing the warmer-related bugs (#5155, #15590 or #15194) that we have, I'd like to suggest that we remove the warmer API entirely.

The migration strategy would be the following:

• deprecate the warmer API in 2.x,
• remove warmers and the warmer API entirely in 3.0, and ignore warmer definitions when upgrading from 2.x.

[kimchy]

++ for this plan, I love the fact that we don't need warmers anymore

[Mpdreamz]

I was just looking at taking a stab at #5155 but this is even better 👍

[bpicolo]

@jpountz How are we expected to take care of the use case of pre-building filter caches in upcoming versions? I can't find any docs suggesting that's not a useful case still (it seems very relevant). I also can't find any actual documentation for filter-caches that claims to still be relevant.