Remove custom styling for tabs elements

What does this MR do and why?

Remove custom styling from the LDAP tabs. It caused to show an upper border in after on the 5th tab onward.

Changelog: changed

Screenshots or screen recordings

Before	After

How to set up and validate locally

To see the top border (like the before screenshot above):

1. On master (not in this branch)
2. Apply this patch:

diff --git a/app/views/devise/sessions/new.html.haml b/app/views/devise/sessions/new.html.haml
index c7cbf09d8460..606e34fd1b9a 100644
--- a/app/views/devise/sessions/new.html.haml
+++ b/app/views/devise/sessions/new.html.haml
@@ -10,7 +10,7 @@
 = render_if_exists "layouts/google_tag_manager_body"
 
 #signin-container
-  - if any_form_based_providers_enabled?
+  - if true
     = render 'devise/shared/tabs_ldap', render_signup_link: false
   .tab-content
     - if password_authentication_enabled_for_web? || ldap_sign_in_enabled? || crowd_enabled?
diff --git a/app/views/devise/shared/_tabs_ldap.html.haml b/app/views/devise/shared/_tabs_ldap.html.haml
index 0d7929b6acb8..e5a563ccbe94 100644
--- a/app/views/devise/shared/_tabs_ldap.html.haml
+++ b/app/views/devise/shared/_tabs_ldap.html.haml
@@ -1,7 +1,7 @@
 - show_password_form = local_assigns.fetch(:show_password_form, password_authentication_enabled_for_web?)
 - render_signup_link = local_assigns.fetch(:render_signup_link, true)
 
-%ul.nav-links.new-session-tabs.nav-tabs.nav.nav-links-unboxed{ class: "#{'custom-provider-tabs' if any_form_based_providers_enabled?}" }
+%ul.nav-links.new-session-tabs.nav-tabs.nav.nav-links-unboxed{ class: "#{'custom-provider-tabs' if true}" }
   - if crowd_enabled?
     %li.nav-item
       = link_to _("Crowd"), "#crowd", class: "nav-link #{active_when(form_based_auth_provider_has_active_class?(:crowd))}", 'data-toggle' => 'tab', role: 'tab'

3. Create 5 tabs, by duplicate the li element using the web console:

Screen_Recording_2023-12-22_at_15.14.57

4. On an incognito window, go to /users/sign_in. Sometimes the upper border in the tabs is more visible in medium size screens:

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.