Remove deprecated config options for RUM

[simitt]

In 7.15 several RUM related configuration options have been deprecated. Since they were deprecated throughout the whole 8.x lifecycle, they should be removed in 9.0. All of the removed config options have a replacement available, so we are not taking away functionality. The goal is to have more streamlined configuration options.

The following options should be removed:

• apm-server.rum.event_rate.limit
• apm-server.rum.event_rate.lru_size
• apm-server.rum.allow_service_names

Deprecation has been documented.

[kruskall]

These options have been removed in 8.0 with #6560

cc @simitt maybe we need a new issue to review the docs ? Can we close this one ?

[simitt]

In that case, yes please ensure that the docs are updated accordingly (also at least for current 8.x versions) and close this.

[kruskall]

Opened elastic/observability-docs#4644 to take care of the docs