Remove modes from Preview 2 spec

[pchickey]

We believe there is value in supporting file modes for many applications, but
for the sake of shipping Preview 2, we are out of time to implement these
methods and test a a cross platform implementation of these in two separate
engines.

These functions may come back as part of an additional filesystem interface in
a post Preview 2 patch version 0.2.n, or in Preview 3 or beyond. Until then,
their design is still in the git history.

[pchickey]

Copying this relevant discussion in from bytecodealliance/wasmtime#7510 :

@alexcrichton alexcrichton 3 hours ago
Question on this, if we know we want to add this in the future should we be trying to future-proof this function? For example we could add an options: option with a currently empty resource open-options or something like that.

Alternatively we could add a feature to the component model and WIT for something like keyword arguments with defaults. That's a much longer- term vision though.

@pchickey pchickey 34 minutes ago
I believe that we can eliminate the modes parameter here and let open-at continue to do the reasonable default behavior, and then when users of the re-introduced modes stuff want to opt into a non-default behavior for modes, we can expose a new open-at-modes function that has the additional argument. Then when it comes time to make a breaking change in preview 3 we can consolidate down to just one open-at with mode arguments.

We could get fancier than that, but I expect the above will work fine without introducing much complexity.