Remove unused _replace_nan function

Author: tomwhite

xarray/core/nanops.py

@@ -8,15 +8,6 @@
 from .duck_array_ops import count, fillna, isnull, sum_where, where, where_method
 
 
-def _replace_nan(a, val):
-    """
-    replace nan in a by val, and returns the replaced array and the nan
-    position
-    """
-    mask = isnull(a)
-    return where_method(val, mask, a), mask
-
-
 def _maybe_null_out(result, axis, mask, min_count=1):
     """
     xarray version of pandas.core.nanops._maybe_null_out