Add delete system variables

[Ashish-Kumar-Dash]

Summary

Implements the z/OSMF "Delete system variables" REST API (closes #508 ): DELETE /variables/rest/1.0/systems/.

Changes

• VariableDelete (methods) — new API class with two operations: - delete(sysplexName, systemName) — bodyless request that deletes the entire system variable pool
  • delete(sysplexName, systemName, variableNames) — sends a JSON array body to delete specific named variables
• DeleteJsonZosmfRequest (rest) — enhanced to optionally carry a request body. setBody now stores the body (instead of throwing) and executeRequest attaches it only when present, so existing bodyless deletes are unchanged.

Notes

• The IBM doc example path had a typo (.../variables/systems/...); used the canonical .../systems/... consistent with the existing VariableConstants.RESOURCE.
• The whole-pool delete clears any prior body so a reused request instance always issues a true bodyless request.
• Empty variableNames is rejected with IllegalArgumentException (the API treats [] as a no-op; rejecting guides correct usage).

[frankgiordano]

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