Remove cf-encoding workaround

[aulemahal]

The encoding of cftime coordinates within sdba's map_blocks could be removed as the upstream issue in xarray seems to be fixed : pydata/xarray#5701. I don't know how it got fixed, but I believe the snippet below (and some other) and the xclim options can be safely removed. Or I guess the option should stay for 1 or 2 versions and emit a warning.

https://github.com/Ouranosinc/xclim/blob/7acc410dc34f878ace5230b3e34ed4eee25dae03/xclim/sdba/base.py#L675-L683

FYI for xsdba, @coxipi