Prevents entire data panel being collapsed when a stock stops trading.

The call to `Panel.dropna` after the fillna was deleting all values,
if a stock stopped trading mid run and thus provided volume 0.

i.e. if any sid had 0 non-null values the entire panel of frames
would be truncated.

It's possible to avoid the collapse via by adding the `how='all'` flag
to `dropna`, however with the current tick based creation of the panel,
the `dropna` with `how='all'` should be functionally equivalent to
not dropping at all.

The dropna has been dropped in favor of leaving the drop to algorithm
code.