Remove unused methods from MultiSpan

[sophiajt]

Removed a couple of unused methods from MultiSpan. I thought about batching this with some other changes but wasn't sure when I'd get around to them, so PR for a tiny fix instead.

This can be rollup'd.

[rust-highfive]

r? @aturon

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[sophiajt]

@bors r? @nikomatsakis

[sanxiyn]

@bors r+ rollup

[bors]

📌 Commit f7019a4 has been approved by sanxiyn

[mcarton]

This PR seems to have removed the only way to create a MultiSpan with more that one primary span (the only two ways to create one are now MultiSpan::new() and impl From<Span> for MultiSpan). Since primary_spans is a vector and used everywhere as if there can be more than one primary span (it's sorted, its min is taken, etc) I wonder whether this is intentional. If so, more cleanup should be done and primary_spans should be made an option.

This also breaks Clippy (https://github.com/Manishearth/rust-clippy/issues/1120, https://github.com/Manishearth/rust-clippy/issues/1119) without a simple way to fix it, so I'd like to know whether I can hope getting that method back or should remove the broken Clippy feature.

[sophiajt]

@mcarton I joined the discussion on https://github.com/Manishearth/rust-clippy/pull/1119 to see if there are alternate fixes for clippy. Since MultiSpan isn't being used in the compiler in this way, I wanted to continue to simplify it. As you point out, there's more there to be done.