Remove Synth Finance references (#47)

* Remove Synth Finance integration

* Linter noise

* Fix failing (old) test, use it for Twelve Data

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Signed-off-by: Juan José Mata <juanjo.mata@gmail.com>

test/interfaces/security_provider_interface_test.rb

@@ -27,7 +27,7 @@ module SecurityProviderInterfaceTest
 
       assert response.success?
       assert response.data.first.date.is_a?(Date)
-      assert_equal 147, response.data.count # Synth won't return prices on weekends / holidays, so less than total day count of 213
+      assert_equal 147, response.data.count
     end
   end