Remove column_index from MathesarColumn class

[mathemancer]

Problem

Currently, column operations use column_index to identify columns. This is brittle, and leads to bugs. We've decided to change to attnum. Once this is done, the column_index attribute will be extraneous.

Proposed solution

We should remove the column_index attribute of the MathesarColumn class in db/columns/base.py and make sure nothing breaks.

Additional context

Blocked by:

• Column selecting operations should use attnum #992
• Column dropping operation should use attnum #993
• Column alteration operations should use attnum #994
• Column creation and copying should use attnum #995

[kgodey]

I realized that column index is not extraneous, it should still be used by the frontend to order columns for display. See: #1185.

I do think we need to make sure nothing in the db module depends on the column_index attribute of the MathesarColumn class, but I don't think we should remove it, we need to be able to send it to the frontend easily.

[mathemancer]

The attnum of a column is always in the same order as the index. I.e., if you order by attnum, it will always be the same as if you order by column index. If the API needs to return an index instead of attnum, I think we should do that as close to the actual API as possible.

[kgodey]

Okay, makes sense. Based on conversations in #1185 and #1186, I don't think the API needs to return an index, just order by attnum, so we can go ahead and remove the index.