Remove unused _api import

The combination of matplotlib#30014 and matplotlib#30015 made this import redundant, but as
each PR was indepenently tested and reviewed, nothing was evident until
both were merged.

Author: QuLogic

lib/mpl_toolkits/axisartist/grid_helper_curvelinear.py

@@ -7,7 +7,6 @@
 import numpy as np
 
 import matplotlib as mpl
-from matplotlib import _api
 from matplotlib.path import Path
 from matplotlib.transforms import Affine2D, Bbox, IdentityTransform
 from .axislines import (