remove duplicated configuration (#11283)

### Motivation
Remove duplicated configuration when `createBkClientConfiguration` 

Currently, the configuration for BookKeeper client's `NumWorkerThreads` was duplicated in 
https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/BookKeeperClientFactoryImpl.java?#L115
and 
https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/BookKeeperClientFactoryImpl.java?#L126

So `NumWorkerThreads` is  set to 1 finally which is not expected.
### Modifications

remove the duplicated hardcode for `setNumWorkerThreads` to make `NumWorkerThreads` configurable

pulsar-broker/src/main/java/org/apache/pulsar/broker/BookKeeperClientFactoryImpl.java

@@ -123,7 +123,6 @@ ClientConfiguration createBkClientConfiguration(ServiceConfiguration conf) {
         bkConf.setStickyReadsEnabled(conf.isBookkeeperEnableStickyReads());
         bkConf.setNettyMaxFrameSizeBytes(conf.getMaxMessageSize() + Commands.MESSAGE_SIZE_FRAME_PADDING);
         bkConf.setDiskWeightBasedPlacementEnabled(conf.isBookkeeperDiskWeightBasedPlacementEnabled());
-        bkConf.setNumWorkerThreads(1);
 
         if (StringUtils.isNotBlank(conf.getBookkeeperMetadataServiceUri())) {
             bkConf.setMetadataServiceUri(conf.getBookkeeperMetadataServiceUri());