Remove fomantic header module (#30033)

Likely still a few useless classes left, but I think I at least don't
have missed any.

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Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/index.css

@@ -14,6 +14,7 @@
 @import "./modules/svg.css";
 @import "./modules/flexcontainer.css";
 @import "./modules/message.css";
+@import "./modules/header.css";
 
 @import "./shared/flex-list.css";
 @import "./shared/milestone.css";