go/cache: use generics and remove unused API

[mdlayher]

Description

The two remaining uses of the old cache API use a fixed cost of 1 and can still benefit from adding a type-safe generic interface.

Remove a couple of entirely unused methods. There are several methods at this point regarding hit/miss/eviction that are otherwise unused outside of tests but I've left them at this point for the sake of coverage.

Related Issue(s)

n/a, cleanup.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

n/a

[vitess-bot]

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