docs: don't advise to use loading="eager" on images because it's already the default

[bfontaine]

Hello,

loading="eager" is already the default, so adding it explicitely doesn’t change anything. This PR removes this advice from the docs.

See:

• https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/loading#value
• https://stackoverflow.com/a/77043840/735926

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13228-svelte.vercel.app/

this is an automated message

[eltigerchino]

Thank you!