Remove deprecated remove_field function with 7.0

## What?

The final piece missing after **https://github.com/Graylog2/graylog2-server/issues/19287#issuecomment-2603014887**: The deprecated remove_field function needs to be removed with 7.0.
Aside of the actual removal of the function, we want to add some UI guidance for users when their functions break after the upgrade.

## Why?

To address potential performance issues from https://github.com/Graylog2/graylog2-server/issues/19098. We already took the necessary steps to prepare this by deprecating the function this in the past.

## Your Environment

* Graylog Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: