Revert "Update backend selection logic"

[maarten-ic]

This reverts commit 33e6b7e.

Updated backend selection logic can lead to unexpected situations when making a typo in the URI, for example:

with imas.DBEntry("imas;hdf5?path=xyz", "w") as entry:
    entry.put(ids)

Will now happily create a netCDF file imas;hdf5?path=xyz with the develop branch. Changing this logic back will raise an exception by imas_core.