Remove the service worker (#25010)

It's been disabled by default since 1.17
(https://github.com/go-gitea/gitea/pull/18914), and it never really
delivered any benefit except being another cache layer that has its own
unsolved invalidation issues. HTTP cache works, we don't need two cache
layers at the browser for assets.

## ⚠️ BREAKING

You can remove the config `[ui].USE_SERVICE_WORKER` from your `app.ini`
now.

modules/setting/ui.go

@@ -32,7 +32,6 @@ var UI = struct {
 	CustomEmojis          []string
 	CustomEmojisMap       map[string]string `ini:"-"`
 	SearchRepoDescription bool
-	UseServiceWorker      bool
 	OnlyShowRelevantRepos bool
 
 	Notification struct {
@@ -136,7 +135,6 @@ func loadUIFrom(rootCfg ConfigProvider) {
 	UI.ShowUserEmail = sec.Key("SHOW_USER_EMAIL").MustBool(true)
 	UI.DefaultShowFullName = sec.Key("DEFAULT_SHOW_FULL_NAME").MustBool(false)
 	UI.SearchRepoDescription = sec.Key("SEARCH_REPO_DESCRIPTION").MustBool(true)
-	UI.UseServiceWorker = sec.Key("USE_SERVICE_WORKER").MustBool(false)
 
 	// OnlyShowRelevantRepos=false is important for many private/enterprise instances,
 	// because many private repositories do not have "description/topic", users just want to search by their names.