While posting this question, I meant to put the constructive-mathematics tag, but I accidentally just typed "con", thereby creating a "con" tag in MathOverflow. How can I delete it?
Simply remove the tag from your question, unless someone else uses the tag it will be removed in 24 hours. Here is a very similar question from meta.SO: I accidentally created a tag. How do I delete it?
More details on pruning of low-usage tags:
The process removing tags having no questions is running once in 24 hours, see How can we get rid of misspelled and unused (or "zombie") tags?
The process removing tag containing only one question is run once in a month and it removes tags that are at least 6 months old, see this answer on meta.SO. The exception is that single occurrence tags which have a tag-wiki are not removed by this proces.