Simplify the handling of extrapolated values.

As a result of the earlier upgrade of `iris.experimental.regrid.regrid_bilinear_rectilinear_src_and_grid()` to use the SciPy interpolator `_RegularGridInterpolator`, it is no longer necessary to detect out-of-range values and set them to NaN. Instead, this can be done automatically by setting the appropriate `bounds_error` and `fill_value` values on the `_RegularGridInterpolator` instance.