`ZpoolHealth`: Don't implement `FromStr`

[jgallagher]

I agree with replacing FromStr. That looks like it's parsing the CLI tool output format.

Originally posted by @bnaecker in #10158 (comment)

[jgallagher]

More context from the TODO:

omicron/sled-agent/types/versions/src/impls/inventory.rs

Lines 914 to 916 in f4dd98d

	// TODO-correctness `ZpoolHealth` implements both `FromStr` and `Display`, but
	// they aren't symmetric - we should replace one of these (probably `FromStr`?)
	// with an explicitly-named method.

and more general background is in #10010, which made similar changes to another type.