Deprecate custom agent service deployer

[mrodm]

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:

elastic-package/docs/howto/system_testing.md

Line 113 in 9c8276d

### Agent service deployer

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.