DEV: Remove assertion causing flaky test (#26075)

Why this change?

Assertions against the database in a system test is not reliable because
the request sent from the client side may not have been processed when
the query to the database has been run.

The test was added to prevent a regression for 63119144ff
but it turns out that the test will still prevent the regression even if
we do not assert against the state in the database.

spec/system/admin_customize_themes_spec.rb

@@ -69,8 +69,6 @@ describe "Admin Customize Themes", type: :system do
       expect(color_scheme_settings.find(".setting-value")).to have_content(color_scheme.name)
       expect(color_scheme_settings).not_to have_css(".submit-edit")
       expect(color_scheme_settings).not_to have_css(".cancel-edit")
-
-      expect(theme.reload.color_scheme_id).to eq(color_scheme.id)
     end
   end