remove uses of Ord::clamp in scalbn

[quaternic]

Avoid using Ord::clamp in the f16-specific part of the generic scalbn.

fixes #1046

It turned out to be redundant anyway, as both callsites follow a pattern like

if n < negative_val {
    let foo = (n + positive_val).clamp(negative_val, positive_val);
}

and n < negative_val < 0 implies n + positive_val < positive_val.

[zmodem]

Is this ready for review? Do we need to add someone?

[tgross35]

Thank you!

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.