[UnifiedFieldList] Remove the deprecated logic around Smart fields

[jughosta]

It was used for Log Explorer in the past but now it's deprecated. We can make the package lighter by removing Smart Fields support from the UnifiedFieldList and possible Discover utils.

Got reminded about it because of #217034

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[davismcphee]

Thanks for creating the issue, and agreed. I've been meaning to create a similar issue for a while but never got around to it... I think we can probably drop it all from both Unified Field List and Discover since it's mostly dead code, aside from maybe a bit that's still used for the summary column.