BUG: Revert "Merge ability to specify timing of fills."

This reverts commit e3a9ca27b177a45ee4ae0f7f6d808b7e410fd6ac, reversing
changes made to 3d8bdeb42971d5507fee221ccac00295612ff076.

Conflicts:
	zipline/gens/tradesimulation.py

The aforementioned change needs a revert because it caused a 'doubling'
of orders, since the portfolio is not updated until after handle_data
is called a second time after an order has been processed.

The flexibility of fill_delay is still desired, but remove for now,
favoring reverting back to existing behavior over trying ot fix the
fill_delay logic.