do not check brentq is NotImplemented

* related to pvlib#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

Author: mikofski

pvlib/singlediode.py

@@ -209,8 +209,6 @@ def fv(x, v, *a):
         return bishop88(x, *a)[1] - v
 
     if method.lower() == 'brentq':
-        if brentq is NotImplemented:
-            raise ImportError('This function requires scipy')
         # first bound the search using voc
         voc_est = estimate_voc(photocurrent, saturation_current, nNsVth)
 
@@ -275,9 +273,6 @@ def fi(x, i, *a):
         return bishop88(x, *a)[0] - i
 
     if method.lower() == 'brentq':
-        if brentq is NotImplemented:
-            raise ImportError('This function requires scipy')
-
         # brentq only works with scalar inputs, so we need a set up function
         # and np.vectorize to repeatedly call the optimizer with the right
         # arguments for possible array input
@@ -336,8 +331,6 @@ def fmpp(x, *a):
         return bishop88(x, *a, gradients=True)[6]
 
     if method.lower() == 'brentq':
-        if brentq is NotImplemented:
-            raise ImportError('This function requires scipy')
         # break out arguments for numpy.vectorize to handle broadcasting
         vec_fun = np.vectorize(
             lambda voc, iph, isat, rs, rsh, gamma: