[FR] allow unassigning shortcut

### Description

Let the user unassign an existing shortcut. Don't force them to have shortcuts assigned to every function if they don't want to. It could be as simple as adding a little ❌ button near the ✏️ one, and it would just clear out the key combination.

If the user wants to assign a shortcut to that later, they always can. But right now we only can assign a different key combination.

### Impact

Like many people in the world, I don't use an American QWERTY keyboard layout. So it can happen quite often that pressing a specific combination of keys is needed for me to write a common character for my language. Which can become hard to do when a software catches those key combinations as a function shortcut. So the only solution is to either change the shortcut's key combination to something else, or sometimes to remove the shortcut because you don't need it and prefer saving your shortcut layout for something else.

In Appflowy for example, ⎈ Ctrl⎇ AltE is used to invoke the emoji picker, but I need that very same combination of keys to write a very common character in my native language.

So I went to the shortcuts' editor, and since I don't see any use of that emoji picker for me (Windows' native picker is more furnished and in non-conflicting shortcut), I tried deleting the shortcut, but it's impossible. I had to replace it by a stupid combination that I am sure I will never use in any other occasion:


If I could just delete it, there would be no problem here, in addition to be more user-friendly.

### Additional Context

_No response_