cards: remove len calculation

The method update_ad() caluclates the size of the data it had just
read from EF.AD, but the result is never used, lets remove that
line

Change-Id: Id38c0dc725ab6874de3ea60132482a09372abe9e

pySim/cards.py

@@ -149,7 +149,6 @@ class Card(object):
 			raise RuntimeError('unable to calculate proper mnclen')
 
 		data = self._scc.read_binary(EF['AD'], length=None, offset=0)
-		size = len(data[0]) // 2
 		content = data[0][0:6] + "%02X" % mnclen
 		data, sw = self._scc.update_binary(EF['AD'], content)
 		return sw