[APM] Remove `apm_oss.indexPattern` from the docs

[gbamparop]

Remove apm_oss.indexPattern from the documentation as it is no longer used.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[gbamparop]

Hi @bmorelli25, could you have a look at this issue please?

[bmorelli25]

@gbamparop Just the apm_oss.indexPattern setting? Can you provide more context as to why this should be removed? We currently link to this setting in the APM API docs: Customize the APM index pattern.

[bmorelli25]

I see the setting was removed from the docs in #114139, and other settings were updated in #113364, but I'm still not sure what context is for this removal.

Should we only mention the Kibana update object API as a way to update the default APM index pattern? Or should the entire Customize the APM index pattern topic be removed?

[bmorelli25]

Opened the two PRs ⬆️. Questions still stand but can be addressed in the PRs. Thanks for bringing this to my attention.