chore(ui): remove unused fomantic font size classes (#7705)

One can grep `massive` and `huge` on the current codebase and not find any evidence of these classes being used anywhere in the UI, probably because there's a good reason for this. So these rules are a dead weight.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7705
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

web_src/fomantic/_site/globals/site.variables

@@ -7,6 +7,7 @@
 @disabledOpacity: var(--opacity-disabled);
 @linkHoverUnderline: underline;
 @pageOverflowX: visible;
+@variationAllSizes: mini, tiny, small, large;
 
 @variationAccordionInverted: false;
 @variationBreadcrumbInverted: false;