Simplify Cohort default

[tavinathanson]

Seems very easy to accidentally forget the following lines at the beginning of functions:

filter_fn = first_not_none_param([filter_fn, cohort.filter_fn], no_filter)
normalized_per_mb = first_not_none_param([normalized_per_mb, cohort.normalized_per_mb], False)

...and thereby not use the Cohort default.

[tavinathanson]

Indeed, @jburos ran into this very issue: https://github.com/hammerlab/cohorts/issues/120#issue-171240320