STY: Remove warning when ordering zero shares.

Many algorithms that use the new order methods like order_target()
will legitimately try to order 0 shares many times. The printed
warning at every turn is quite annoying and too verbose. We do not
display it on Quantopian either so I'm removing it here as well.

zipline/finance/blotter.py

@@ -100,12 +100,7 @@ class Blotter(object):
         amount = int(almost_equal_to(amount))
 
         # just validates amount and passes rest on to TransactionSimulator
-        # Tell the user if they try to buy 0 shares of something.
         if amount == 0:
-            zero_message = "Requested to trade zero shares of {psid}".format(
-                psid=sid
-            )
-            log.debug(zero_message)
             # Don't bother placing orders for 0 shares.
             return
         elif amount > self.max_shares: