Deprecate cutoff_frequency for ES7

[orangejulius]

Looking at the deprecation logs for a running ES7 cluster, the only messages are the following:

[2020-01-16T23:20:05,749][WARN ][o.e.d.c.ParseField       ] Deprecated field [cutoff_frequency] used, replaced by [you can omit this option, the [multi_match] query can skip block of documents efficiently if the total number of hits is not tracked]
[2020-01-16T23:20:23,850][WARN ][o.e.d.c.ParseField       ] Deprecated field [cutoff_frequency] used, replaced by [you can omit this option, the [match] query can skip block of documents efficiently if the total number of hits is not tracked]

Looks like after all the work we did to use cutoff_frequency, it's basically no longer needed in ES7. We might as well remove it if we get the chance.

[missinglink]

related #134

[missinglink]

we introduced this in pelias/api#1213, however since elasticsearch 7, the equivalent behaviour of cutoff_frequency is handled internally without configuration.

just wanted to check this was safe to remove and the answer is yes, es6 and below will suffer a perf regression but we no longer support those versions so it's safe to remove.