[SPARK-2144] ExecutorsPage reports incorrect # of RDD blocks

This is reproducible whenever we drop a block because of memory pressure.

When this happens, the StorageStatus should remove the block from its mapping of BlockId to BlockStatus. Instead, it simply replaces the BlockStatus with one that has a storage level of `StorageLevel.NONE`. This PR makes sure that we remove the block from the map if it is no longer cached.