Revert "Track last allocations to ensure forward-progress"

This reverts commit 9f553c9a9752071129f6e8a31535829c5e9a0d91.

We have observed a problem with this patch where nodepool may
attempt to allocate more than the configured quota.

Steps to reproduce:

* Nodepool has a very high (>2x quota) request load
* Restart nodepool
* First pass through allocator appears normal and allocates up to quota
* Second pass allocates an additional $quota worth of nodes, all from
  the last-defined provider
* Repeats until request load is satisfied

This caused us to request thousands of nodes from our providers at
once.

Change-Id: I08e5fd2de668cc2fc2d68bb1bf09f2d725f82c7f