Remove outdated RunUser logic (#37180)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.

custom/conf/app.example.ini

@@ -41,10 +41,10 @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;; App name that shows in every page title
-APP_NAME = ; Gitea: Git with a cup of tea
+;APP_NAME = Gitea: Git with a cup of tea
 ;;
 ;; RUN_USER will automatically detect the current user - but you can set it here change it if you run locally
-RUN_USER = ; git
+;RUN_USER =
 ;;
 ;; Application run mode, affects performance and debugging: "dev" or "prod", default is "prod"
 ;; Mode "dev" makes Gitea easier to develop and debug, values other than "dev" are treated as "prod" which is for production use.