remove temporary enforcement of netcdf4 engine for opendap files

[veenstrajelmer]

Follow-up of #1265. In the linked PR, the xarray engine was temporarily set to netcdf4 since xarray 2025.09.1 onwards have h5netcdf as default engine and this does not handle opendap files properly. This temporary enforcement can be removed after pydata/xarray#10810 is fixed.

Todo:

• revert this temporary enforcement including TODO comments
• remove temporary entry in whats-new.md
• consider excluding some xarray versions in pyproject.toml

[veenstrajelmer]

The enforcement was not implemented since this issue will be fixed in xarray 2025.10.1 instead: pydata/xarray#10828