Remove unnecessary let in spec

spec/lib/gitlab/database_importers/common_metrics/prometheus_metric_spec.rb

@@ -3,17 +3,14 @@
 require 'rails_helper'
 
 describe Gitlab::DatabaseImporters::CommonMetrics::PrometheusMetric do
-  let(:existing_group_titles) do
-    ::PrometheusMetricEnums.group_details.each_with_object({}) do |(key, value), memo|
-      memo[key] = value[:group_title]
-    end
-  end
-
   it 'group enum equals ::PrometheusMetric' do
     expect(described_class.groups).to eq(::PrometheusMetric.groups)
   end
 
   it '.group_titles equals ::PrometheusMetric' do
+    existing_group_titles = ::PrometheusMetricEnums.group_details.each_with_object({}) do |(key, value), memo|
+      memo[key] = value[:group_title]
+    end
     expect(Gitlab::DatabaseImporters::CommonMetrics::PrometheusMetricEnums.group_titles).to eq(existing_group_titles)
   end
 end