Luckily, that icon is not used on the site, since the settings icon is not visible. The png icons are deprecated, only a few search templates have them – others use SVG icons. This must be something I've missed, apologies for that.
I've logged in with the credentials you gave and changed the settings icon to another one. This won't change anything visually, but a new CSS file is generated, now without the sett0.png icon reference.
Once you clear your CDN and page cache, the search stylesheet should no longer contain this icon, and the pagespeed should not report it anymore 😉Best,
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