fix(content): ignore non-monetization links when `<link>` removed

[sidvishnoi]

Context

Visiting some sites that remove link tags on load (in this case Google calendar) led to following error in console:

Uncaught (in promise) Error: Could not find details for monetization node <link rel="icon" id="favicon" type="image/x-icon" href="https://calendar.google.com/googlecalendar/images/favicons_2020q4/calendar_1.ico">
    at MonetizationLinkManager.onRemovedLink (content.js:3456:15)
    at content.js:3347:39
    at NodeList.forEach (<anonymous>)
    at MonetizationLinkManager.onWholeDocumentObserved (content.js:3345:31)

Changes proposed in this pull request

Ignore removed link nodes that aren't relevant.

[github-actions[bot]]

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