Remove the never-public org.metricshub.winrm.backend system property from the code

[bertysentry]

Summary

Remove the org.metricshub.winrm.backend system property and its backend-selection logic from the code entirely.

Rationale

The property was never released to the public. It was introduced during 2.0.0 development purely as a transitional guard around the removal of the legacy CXF backend: it defaults to light (the only implementation) and rejects cxf with an error. Since no public release ever exposed it, no user can have it set, so the property serves no purpose and only adds dead branches and API surface.

The documentation references have already been removed (site docs, README.md, CHANGELOG.md). This issue tracks removing it from the code.

Scope

• WinRMExecutorFactory
  • Remove the public BACKEND_PROPERTY constant and the LIGHT / CXF string constants.
  • Simplify createInstance(...) to delegate straight to LightWinRMService.createInstance(...) (drop the System.getProperty(...) read and the light / cxf / unknown-value branching).
  • Update the class Javadoc that currently documents the property.
• WinRMExecutorFactoryTest
  • Remove/adjust the tests that exercise the property (the cxf rejection, the unknown-value rejection, and any System.setProperty/clearProperty handling around org.metricshub.winrm.backend).

Acceptance criteria

• No reference to org.metricshub.winrm.backend remains anywhere in src/ (verify with a repo grep).
• WinRMExecutorFactory.createInstance(...) still returns a LightWinRMService-backed executor with the same behavior.
• mvn verify is green.