Removing the remove_category function

[josenavas]

I don't think the remove_category function is needed, so I'm removing it from the code base.

The reasons why I think this function should be removed:

• It is not used in any place in the code base
• It has a comment that says "This operation may invalidate another user's perspective on the table", but no checks o safe mechanism is put in place.
• It is buggy: it is dropping a column from the dynamic table, but is not updating the study-columns table.
• Leaves the filesystem in an inconsistent state: once the column is removed, the sample template file and the qiime mapping file is not being re-generated, so the files do not represent the data in the DB
• Before removing any column in the metadata templates, multiple checks has to be done (i.e. it has been used in an analysis).

Since it is clear that a lot of though has to be put in this function, I think it is safer to just remove it for now and put it back once the functionality is needed and we actually have a design for all the mechanisms to notify users once a template has been changed in this way. Note that IMOO, adding new columns is not a big deal, however, removing columns is another story.

[coveralls]

Coverage decreased (-0.02%) to 79.1% when pulling eeeef3d on josenavas:drop-remove-category into 2199bdd on biocore:master.

[ElDeveloper]

👍 on the basis that it is not being used anywhere and its use could lead to an inconsistent state of the system.