do not check brentq is NotImplemented (#593)
do not check brentq is NotImplemented (#593)

* related to #556 
* if brentq can't be imported it will raise a `NotImplementedError` alread
* lgtm says it's better for a callable to have a consistent API
* that's why we changed it the not-imported `brentq` from `NotImplemented` to a callable, b/c `brentq` is usually callable
* but we forgot the original code has a check if `brentq` is `NotImplemented`, so we can remove that now
rubennj/pvlib-python,cwhanse/pvlib-python,alorenzo175/pvlib-python,wholmgren/pvlib-python,mikofski/pvlib-python,anomam/pvlib-python,pvlib/pvlib-python