Remove redundant theme class for gl_tabs_nav

Now that we depend on @gitlab/ui@35 thanks to
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79430,
theme-specific classes like *-indigo are no longer needed on tabs.

changelog: fixed
See https://gitlab.com/groups/gitlab-org/-/epics/7401 for more details.

app/assets/javascripts/tabs/constants.js

-export const ACTIVE_TAB_CLASSES = Object.freeze([
-  'active',
-  'gl-tab-nav-item-active',
-  'gl-tab-nav-item-active-indigo',
-]);
+export const ACTIVE_TAB_CLASSES = Object.freeze(['active', 'gl-tab-nav-item-active']);
 
 export const ACTIVE_PANEL_CLASS = 'active';
 

app/helpers/tab_helper.rb

@@ -33,7 +33,7 @@ module TabHelper
   #
   def gl_tab_link_to(name = nil, options = {}, html_options = {}, &block)
     link_classes = %w[nav-link gl-tab-nav-item]
-    active_link_classes = %w[active gl-tab-nav-item-active gl-tab-nav-item-active-indigo]
+    active_link_classes = %w[active gl-tab-nav-item-active]
 
     if block_given?
       # Shift params to skip the omitted "name" param

spec/helpers/tab_helper_spec.rb

@@ -45,7 +45,7 @@ RSpec.describe TabHelper do
     end
 
     it 'creates an active tab with item_active = true' do
-      expect(helper.gl_tab_link_to('Link', '/url', { item_active: true })).to match(/<a class=".*active gl-tab-nav-item-active gl-tab-nav-item-active-indigo.*"/)
+      expect(helper.gl_tab_link_to('Link', '/url', { item_active: true })).to match(/<a class=".*active gl-tab-nav-item-active.*"/)
     end
 
     context 'when on the active page' do
@@ -54,7 +54,7 @@ RSpec.describe TabHelper do
       end
 
       it 'creates an active tab' do
-        expect(helper.gl_tab_link_to('Link', '/url')).to match(/<a class=".*active gl-tab-nav-item-active gl-tab-nav-item-active-indigo.*"/)
+        expect(helper.gl_tab_link_to('Link', '/url')).to match(/<a class=".*active gl-tab-nav-item-active.*"/)
       end
 
       it 'creates an inactive tab with item_active = false' do