Remove QPlatformInputContext::handlesInputPanelVisibility() after deprecation period

An ownership of input panel visibility was changed from QInputPanel to QPlatformInputContextIn a recent change. The feature could not be enabled immediatelly without breaking existing input methods, so handlesInputPanelVisibility() function was added to query whether the platform input context supports the new functionality. This temporary function should be removed after a safe transition period, but well before the maturization of Qt 5.0.