Remove section on adding custom attributes with account v1 and custom themes (#26858)

Closes #26856

Signed-off-by: stianst <stianst@gmail.com>

docs/documentation/server_development/topics.adoc

@@ -4,7 +4,6 @@ include::topics/themes.adoc[]
 include::topics/themes-selector.adoc[]
 include::topics/themes-resources.adoc[]
 include::topics/locale-selector.adoc[]
-include::topics/custom-attributes.adoc[]
 include::topics/identity-brokering.adoc[]
 include::topics/identity-brokering/tokens.adoc[]
 include::topics/identity-brokering/account-linking.adoc[]

docs/documentation/server_development/topics/custom-attributes.adoc

@@ -1,39 +0,0 @@
-
-[[_custom_user_attributes]]
-== Custom user attributes
-
-You can add custom user attributes to the registration page with a custom theme.
-
-=== Registration page
-
-Use this procedure to enter custom attributes in the registration page.
-
-.Procedure
-
-. Copy the template `themes/base/login/register.ftl` to the login type of your custom theme.
-
-. Open the copy in an editor.
-+
-For example, to add a mobile number to the registration page, add the following snippet to the form:
-+
-[source,html]
-----
-<div class="form-group">
-   <div class="${properties.kcLabelWrapperClass!}">
-       <label for="user.attributes.mobile" class="${properties.kcLabelClass!}">Mobile number</label>
-   </div>
-
-   <div class="${properties.kcInputWrapperClass!}">
-       <input type="text" class="${properties.kcInputClass!}" id="user.attributes.mobile" name="user.attributes.mobile" value="${(register.formData['user.attributes.mobile']!'')}"/>
-   </div>
-</div>
-----
-
-. Ensure the name of the input html element starts with `user.attributes`. In the example above, the attribute will be stored by {project_name} with the name `mobile`.
-
-. To see the changes, make sure your realm is using your custom theme for the login theme and open the registration page.
-
-[role="_additional-resources"]
-=== Additional resources
-
-* See <<_themes,Themes>> for how to create a custom theme.