V14: Remove user-defined.css (#16792)

* chore: remove the `user-defined.css` asset from the RCL

You cannot overwrite an asset in an RCL just like that, as the user-defined.css were intended.

* delete actual file

src/Umbraco.Cms.StaticAssets/umbraco/UmbracoBackOffice/Index.cshtml

@@ -33,7 +33,6 @@
     <meta name="pinterest" content="nopin"/>
     <title>Umbraco</title>
 
-    <link rel="stylesheet" href="@backOfficeAssetsPath/css/user-defined.css" />
     <link rel="stylesheet" href="@backOfficeAssetsPath/css/umb-css.css" />
     <link rel="stylesheet" href="@backOfficeAssetsPath/css/uui-css.css" />
     @await Html.BackOfficeImportMapScriptAsync(JsonSerializer, BackOfficePathGenerator, PackageManifestService)

src/Umbraco.Cms.StaticAssets/umbraco/UmbracoLogin/Index.cshtml

@@ -45,7 +45,6 @@
     <meta name="pinterest" content="nopin"/>
     <title>Umbraco</title>
 
-    <link rel="stylesheet" href="~/umbraco/login/user-defined.css" asp-append-version="true"/>
     <link rel="stylesheet" href="~/umbraco/backoffice/css/uui-css.css" asp-append-version="true" />
     <style>
       body {