Move calculate_ballot_lines task to the right file

We already have a file with the proper budgets namespace.

lib/tasks/budgets.rake

@@ -1,4 +1,11 @@
 namespace :budgets do
+  desc "Regenerate ballot_lines_count cache"
+  task calculate_ballot_lines: :environment do
+    Budget::Ballot.find_each do |ballot|
+      Budget::Ballot.reset_counters ballot.id, :lines
+    end
+  end
+
   namespace :email do
 
     desc "Sends emails to authors of selected investments"

lib/tasks/regenerate_ballot_lines_cache.rake

@@ -1,8 +0,0 @@
-namespace :budgets do
-  desc "Regenerate ballot_lines_count cache"
-  task calculate_ballot_lines: :environment do
-    Budget::Ballot.find_each do |ballot|
-      Budget::Ballot.reset_counters ballot.id, :lines
-    end
-  end
-end