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It might seem a bit redundant, but that's how I would do it.
(I'm assuming you want to keep everything except the position array)
1- Define a new message. I'm defining it in package myPkg and calling it newMsg.

Header header  
string[] name
float64[] velocity
float64[] effort

2- Write a node that on receiving a message (callback), publishes the data you want.

import rospy  
from myPkg.msg import newMsg  
from sensor_msgs.msg import JointState  

def callback(msg):  
       out.header = msg.header  
       out.velocity = msg.velocity  
       out.effort = msg.effort  
       pub.publish(out) 


rospy.init_node('Scraper')  
sub = rospy.Subscriber('in', JointState, callback)  
pub = rospy.Publisher('out', newMsg)

It might seem a bit redundant, but that's how I would do it.
(I'm assuming you want to keep everything except the position array)
1- Define a new message. I'm defining it in package myPkg and calling it newMsg.

Header header  
string[] name
float64[] velocity
float64[] effort

2- Write a node that on receiving a message (callback), publishes the data you want.

import rospy  
from myPkg.msg import newMsg  
from sensor_msgs.msg import JointState  

def callback(msg):  
       out.header = msg.header  
       out.velocity = msg.velocity  
       out.effort = msg.effort  
       pub.publish(out) 


rospy.init_node('Scraper')  
sub = rospy.Subscriber('in', JointState, callback)  
pub = rospy.Publisher('out', newMsg)
newMsg)  
rospy.spin()