[Chore] IndexPrefix option in libbeat settings is never used by any beat type

[IndexPrefix](https://github.com/khushijain21/beats/blob/logstash-metadata/libbeat/cmd/instance/settings.go#L33) setting is never used by any beat. Instead we always fall back to default value here - which is the same as the name of the beat [here](https://github.com/khushijain21/beats/blob/logstash-metadata/libbeat/cmd/instance/beat.go#L223-L225)

I think its a good idea to simplify this by removing IndexPrefix.

Had to go down a rabbit hole only to find out IndexPrefix and beatname are always exactly the same

Start by reading IndexPrefix in libbeat/cmd/instance/settings.go and the defaulting logic in libbeat/cmd/instance/beat.go around the linked lines. Search the Beats repository for other references, then inspect the relevant libbeat tests; done means the unused setting is removed without changing the effective beat-name index behavior.

Written by the indexing model from the issue text.