Remove unused algorithm methods

Two maintenance changes that were made while working on a bug in Quantopian-space:

Removes unused method `raw_positions` and `raw_orders` from `TradingAlgorithm`.
These methods existed to help support features in the Quantopian codebase,
which no longer use the existing interface.

We guarantee that `TradingAlgorithm.updated_portfolio` is called at least once
between each perf message emission.  This change moves that guaranteed call
into `AlgorithmSimulator.get_message` and adds a comment to make it clear why
updated_portfolio is being called.