Remove automatic setting of use_src_mask in regrid

[sloosvel]

Describe the bug
#2070 introduced a way to automatically set ùse_src_mask in the regridder in order to be able to regrid datasets with discontiguous coordinates. The problem is that iris.util.find_discontiguities realises the aux coordinates. Which leads to failures when using dask distributing during the saving because of the arrays that got realized.

Maybe for now it would be best to just set this parameter in the recipe, while there is not any other way to check for discontinuities in the coordinates.