[expr.add] Simplify wording for pointer subtraction

[eisenwave]

Fixes #8564.

In short, the goal is to harmonize with the wording in https://eel.is/c++draft/expr.sizeof

We already say in in <stddef.h> (C) and in [supper.types.layout] that ptrdiff_t is implementation-defined, and saying it a third time here is just silly.

[frederick-vs-ja]

I'm a bit uneasy about the current approach, because it seemingly makes some core language design specified in the library wording.

[eisenwave]

We could also harmonize it the other way around, which is to remove all that wording from [support.types.layout] and pull it into core wording, but that seems like the bigger change.

In any case, it seems wrong to define what ptrdiff_t is on the fly, in the middle of describing pointer subtraction. It should have some dedicated paragraph. We don't do it like that for std::size_t, std::float32_t, or any other library aliases for core types either.