Remove keg-only restriction for nss

[swrobel]

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• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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The Firefox issue linked as the reason for this being keg-only has long since been closed.

[fxcoudert]

Sounds good, merging.