Drop SSPI auth support and more Windows files (#7148)

## Dropping SSPI auth support

SSPI authentication relied on Microsoft Windows support, removal started in https://codeberg.org/forgejo/forgejo/pulls/5353, because it was broken anyway. We have no knowledge of any users using SSPI authentication. However, if you somehow managed to run Forgejo on Windows, or want to upgrade from a Gitea version which does, please ensure that you do not use SSPI as an authentication mechanism for user accounts. Feel free to reach out if you need assistance.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7148
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Otto Richter <otto@codeberg.org>
Co-committed-by: Otto Richter <otto@codeberg.org>

routers/web/auth/auth.go

@@ -164,7 +164,6 @@ func SignIn(ctx *context.Context) {
 	ctx.Data["SignInLink"] = setting.AppSubURL + "/user/login"
 	ctx.Data["PageIsSignIn"] = true
 	ctx.Data["PageIsLogin"] = true
-	ctx.Data["EnableSSPI"] = auth.IsSSPIEnabled(ctx)
 	ctx.Data["EnableInternalSignIn"] = setting.Service.EnableInternalSignIn
 
 	if setting.Service.EnableCaptcha && setting.Service.RequireCaptchaForLogin {
@@ -190,7 +189,6 @@ func SignInPost(ctx *context.Context) {
 	ctx.Data["SignInLink"] = setting.AppSubURL + "/user/login"
 	ctx.Data["PageIsSignIn"] = true
 	ctx.Data["PageIsLogin"] = true
-	ctx.Data["EnableSSPI"] = auth.IsSSPIEnabled(ctx)
 	ctx.Data["EnableInternalSignIn"] = setting.Service.EnableInternalSignIn
 	ctx.Data["DisablePassword"] = !setting.Service.EnableInternalSignIn