Remove `apm_user` from testing user roles

[endorama]

The apm_user role has been removed (elastic/elasticsearch#116712) and latest 9.0 system test are failing.

We are using it here:

apm-server/testing/docker/elasticsearch/users_roles

Line 3 in 7ac2ea6

apm_user:apm_server_user,apm_user_ro

This user is not needed, so we should remove it.

[endorama]

Note that the tests have been updated but we discovered this will change will require updating our documentation to account for the missing permissions when using Tail Based Sampling.

https://github.com/elastic/observability-docs/issues/4718 tracks the required documentation updates.