Refactor setting.Other and remove unused SHOW_FOOTER_BRANDING (#24270)

The `SHOW_FOOTER_BRANDING` came from year 2015, and it seems nobody ever
uses it. It only shows an GitHub icon which seems unrelated to Gitea, it
doesn't do what document says. So, remove it.

## ⚠️ Breaking

Users can now remove the key `[other].SHOW_FOOTER_BRANDING` from their
app.ini.

modules/setting/other.go

@@ -3,20 +3,25 @@
 
 package setting
 
-var (
-	// Other settings
-	ShowFooterBranding         bool
+import "code.gitea.io/gitea/modules/log"
+
+type OtherConfig struct {
 	ShowFooterVersion          bool
 	ShowFooterTemplateLoadTime bool
 	EnableFeed                 bool
 	EnableSitemap              bool
-)
+}
+
+var Other = OtherConfig{
+	ShowFooterVersion:          true,
+	ShowFooterTemplateLoadTime: true,
+	EnableSitemap:              true,
+	EnableFeed:                 true,
+}
 
 func loadOtherFrom(rootCfg ConfigProvider) {
 	sec := rootCfg.Section("other")
-	ShowFooterBranding = sec.Key("SHOW_FOOTER_BRANDING").MustBool(false)
-	ShowFooterVersion = sec.Key("SHOW_FOOTER_VERSION").MustBool(true)
-	ShowFooterTemplateLoadTime = sec.Key("SHOW_FOOTER_TEMPLATE_LOAD_TIME").MustBool(true)
-	EnableSitemap = sec.Key("ENABLE_SITEMAP").MustBool(true)
-	EnableFeed = sec.Key("ENABLE_FEED").MustBool(true)
+	if err := sec.MapTo(&Other); err != nil {
+		log.Fatal("Failed to map [other] settings: %v", err)
+	}
 }