Social Icons Widget / Social Menu Theme Tool: include only SVGs needed

### What's Happening

The following is included in the site's source, whether in use or not:

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### What's Expected

I'd expect only in-use SVGs to be included, and I'd especially expect none of them to be included if the Social Menu is not in use.

originally reported via https://wordpress.org/support/topic/social-widget-feedback/

Locate the Social Icons Widget and Social Menu Theme Tool entry points, then trace where the SVG symbol source shown in the issue is added to the site's source. Verify which icons are configured for use and confirm that the generated source contains only those SVGs; the issue does not name specific files or tests.

Written by the indexing model from the issue text.