change deprecation warning to regular warning (#3449)

Author: lbdreyer

lib/iris/analysis/calculus.py

@@ -26,11 +26,11 @@
 import six
 
 import re
+import warnings
 
 import cf_units
 import numpy as np
 
-from iris._deprecation import warn_deprecated
 import iris.cube
 import iris.coords
 import iris.coord_systems
@@ -91,8 +91,8 @@ def _construct_midpoint_coord(coord, circular=None):
     if circular is None:
         circular = getattr(coord, 'circular', False)
     elif circular != getattr(coord, 'circular', False):
-        warn_deprecated('circular flag and Coord.circular attribute do '
-                        'not match')
+        warnings.warn('circular flag and Coord.circular attribute do '
+                      'not match')
 
     if coord.ndim != 1:
         raise iris.exceptions.CoordinateMultiDimError(coord)