Deprecate custom agent service deployer

Since #2003 , Custom Agent Service deployer was set as deprecated in favor of defining the required Elastic Agents via the Independent Elastic Agents directly in system test configuration files.

This issue is to fully deprecate this Custom Agent Service deployer:

https://github.com/elastic/elastic-package/blob/9c8276dea216a6c8035d919ad1857c5bb870208e/docs/howto/system_testing.md?plain=1#L113

It should be checked what should be removed in at least (not a full list):
- system test runner and tester
- `servicedeployer` module
- `agentdeployer` module
- ...

It could be used some spec version (maybe 3.5?) to fully remove support of this feature.

Start with the deprecation note in docs/howto/system_testing.md around the linked section, then inspect the system test runner and tester, the servicedeployer module, and the agentdeployer module. Determine whether a spec version such as 3.5 is required before removing support. Done means the custom agent service deployer is fully deprecated and the affected system-test paths no longer support it.

Written by the indexing model from the issue text.