Clear sorting information

Hello,

While trying to organize my music, I removed sorting information from tracks because I think I had misunderstood the goal of those sorting fields. Here is an example:

as you can see, the "Sorting Field" is greyed out.

If I attempt to clear that sorting information

As you can see I tried to clear that information by writting a "blank" character. In some other cases I had mistakenly set up that sorting information to " " (blank space).

How on earth do I set that sorting information to BLANK?

Thanks