Remove deprecated legacy_connection_handling setter

This was deprecated in 7.0 and removed in 7.1. This config no longer has
effects since multiple databases config in Rails has to be configured
manually. It is now set to false by default.

spec/dummy/config/environments/test.rb

@@ -7,7 +7,6 @@
   # Settings specified here will take precedence over those in config/application.rb.
 
   config.cache_classes = false
-  config.active_record.legacy_connection_handling = false
 
   # Do not eager load code on boot. This avoids loading your whole application
   # just for the purpose of running a single test. If you are using a tool that