[Operator] Remove gracefulshutdown task

The gracefulshutdown.js task was added to the Operator in order to provide a mechanism for the server to be shutdown correctly before ISPN-13141 was fixed on the server. ISPN-13141 was fixed in DG 8.3.0 and so is no longer relevant, however if calls to server shutdown fails then script execution is still attempted. DG 8.4.x transitioned to Java 17 which no longer contains a JS engine, consequently attempts to execute gracefulshutdown.js result in the below error which masks the underlying reason why the initial shutdown call failed:

[org.infinispan.interceptors.impl.InvocationContextInterceptor] ISPN000136: Error executing command PutKeyValueCommand on Cache '___script_cache', writing keys [___org.infinispan.operator.gracefulshutdown.js] java.lang.IllegalArgumentException: ISPN027510: Cannot find an appropriate script engine for script '___org.infinispan.operator.gracefulshutdown.js'

To prevent this we should simply remove the {{gracefulshutdown.js} task and corresponding controller logic from the Operator.