clear
Truncate one content entry's tile / feature payload while keeping its registration intact — the WebServiceInfo row, the persisted level-set / tile-matrix-set, the display name, the isFloat choice, and any other per-content metadata stay so the caller can re-download tiles without re-configuring the layer.
This is what a "Clear Cache" UI action wants — re-fetch the bytes, keep the layer registered. Use deleteEntry for "remove this layer entirely" instead.
No-op when contentKey doesn't exist or the manager is read-only (in the read-only case, registerWebService-style failures are logged, not thrown).