remove deprecated stuff from std::fmt docs

Fixes #19209
These still reference `:s` and `:u`, perhaps something like:

---

There are various parameters which do require a particular type, however. Namely if the syntax `{:.*}` is used, then the number of characters to print precedes the actual object being formatted, and the number of characters must have the type `uint`. Although a `uint` can be printed with `{}`, it is illegal to reference an argument as such. For example this is another invalid format string:

```
{:.*} {0}
```