Fix decommission UnsupportedOperationException: Remove unsupported

We encountered an "UnsupportedOperationException: Remove unsupported" error when some datanodes were in decommission. The reason of the exception is that datanode.getBlockIterator() returns an Iterator does not support remove, however DatanodeAdminDefaultMonitor#processBlocksInternal invokes it.remove() when a block not found, e.g, the file containing the block is deleted.