mfs: remove unused Flush() from Root

[schomatis]

Towards clarifying #5066.

---

mfs: remove unused `Flush()` from `Root`

The unused `Flush()` function in the `Root` structure helps create the misguided
impression that `Root` implements the `mfs.FSNode` interface (which it doesn't).

[Stebalien]

Really, shouldn't we be calling this function on exit?

[schomatis]

It seems that Root's Flush() was replaced by FlushMemFree() (#4509) which uses the Directory Flush() function (but not Root's).

[Stebalien]

We replaced the existing use of Flush with FlushFreeMem because we wanted to free memory. However, that doesn't mean that Flush isn't otherwise useful (as FlushFreeMem is very unsafe).

[schomatis]

that doesn't mean that Flush isn't otherwise useful

Sure, I'm don't want to imply we shouldn't use Flush(), just that at this point in time this function doesn't seem to be called anywhere in the code base and generates confusion when trying to understand the role of the MFS root, that was my main motivation to remove it, but I'm not against discussing in a separate issue to include it back in the code path. Would you prefer to keep it and close this PR?

[Stebalien]

Would you prefer to keep it and close this PR?

Yes. A lot of the interfaces in go-ipfs are built with the intention of extracting them into a separate library at some point. This is definitely true for MFS.