Remove outdated RunUser logic (#37180)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.

routers/install/install.go

@@ -26,7 +26,6 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/timeutil"
-	"code.gitea.io/gitea/modules/user"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/modules/web/middleware"
 	"code.gitea.io/gitea/routers/common"
@@ -87,15 +86,7 @@ func Install(ctx *context.Context) {
 	form.AppName = setting.AppName
 	form.RepoRootPath = setting.RepoRootPath
 	form.LFSRootPath = setting.LFS.Storage.Path
-
-	// Note(unknown): it's hard for Windows users change a running user,
-	// 	so just use current one if config says default.
-	if setting.IsWindows && setting.RunUser == "git" {
-		form.RunUser = user.CurrentUsername()
-	} else {
-		form.RunUser = setting.RunUser
-	}
-
+	form.RunUser = setting.RunUser
 	form.Domain = setting.Domain
 	form.SSHPort = setting.SSH.Port
 	form.HTTPPort = setting.HTTPPort
@@ -272,13 +263,6 @@ func SubmitInstall(ctx *context.Context) {
 		return
 	}
 
-	currentUser, match := setting.IsRunUserMatchCurrentUser(form.RunUser)
-	if !match {
-		ctx.Data["Err_RunUser"] = true
-		ctx.RenderWithErrDeprecated(ctx.Tr("install.run_user_not_match", form.RunUser, currentUser), tplInstall, &form)
-		return
-	}
-
 	// Check logic loophole between disable self-registration and no admin account.
 	if form.DisableRegistration && len(form.AdminName) == 0 {
 		ctx.Data["Err_Services"] = true