Removes last_calls from SoftLayer.Client

I brought this issue up as a comment to the original pull request:

https://github.com/softlayer/softlayer-python/commit/2724219b6bd0d5826a24682d834f534b2834585d#diff-42ad73fc99a4612e5eafda98f81a1ebaR266

Basically, last_calls is an array that's currently being created on each initialization. It being in the Client class was leaking an implementation of the subclass (TimedClient).