obsolete: alga

[cmungall]

http://purl.obolibrary.org/obo/ENVO_02500019

This is confusing as we would expect to find subtypes of alga here, but there are no subclasses

The note says it should be ceded to pco, but it should be handled the same way as these:

https://github.com/obophenotype/ncbitaxon/issues/45

With the correct union definition it will be pre-reasoned to have the right subclasses and synced with ncbitaxon releases

[wdduncan]

Interestingly, pco has a term alga excluding Cyanobacteria, but not a term for alga.