Remove outdated RunUser logic (#37180)

That logic is from 2014~2015, it unclear why it is necessary or 
whether it is still needed (whether Windows is still special)

The comment "so just use current one if config says default" is not
right anymore: "git" isn't the "default" value of RunUser (Comment out
app.example.ini #15807). The RunUser's value is from current session's
username.

modules/setting/setting.go

@@ -201,7 +201,7 @@ func mustCurrentRunUserMatch(rootCfg ConfigProvider) {
 	if HasInstallLock(rootCfg) {
 		currentUser, match := IsRunUserMatchCurrentUser(RunUser)
 		if !match {
-			log.Fatal("Expect user '%s' but current user is: %s", RunUser, currentUser)
+			log.Fatal("Expect user '%s' (RUN_USER in app.ini) but current user is: %s", RunUser, currentUser)
 		}
 	}
 }