release-22.2: storage: remove Engine interface cruft

[jbowens]

Backport 3/3 commits from #88063.

/cc @cockroachdb/release

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Remove a few methods from the Engine interface type that don't belong.

storage: remove Engine.InMem

Remove storage.Engine's InMem method. This method was a hole in the abstraction. Clients should not need to distinguish between on-disk and in-memory engines.

storage: remove Engine.WriteFile

Remove the WriteFile method on the Engine interface, converting call sites to use the storage/fs.WriteFile function. The Engine itself should only expose primitives, and higher-level utilities like a WriteFile routine can be implemented in terms of those primitives.

storage/fs: move Engine.ReadFile to fs.ReadFile

Move the Engine's ReadFile method off of the Engine interface and into a storage/fs helper function written it terms of existing Engine methods.

Release justification: Non-production code changes and low-risk bug fix

[blathers-crl[bot]]

Thanks for opening a backport.

Please check the backport criteria before merging:

• [x] Patches should only be created for serious issues or test-only changes.
• [x] Patches should not break backwards-compatibility.
• [x] Patches should change as little code as possible.
• [x] Patches should not change on-disk formats or node communication protocols.
• [x] Patches should not add new functionality.
• [x] Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• [ ] There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• [ ] The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• [ ] New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• [ ] The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[jbowens]

tftrs!