Remove Capybara default_set_options for system tests using the Rack Test driver

The next warning is flooding the tests log:

`Options passed to Node#set but the RackTest driver
doesn't support any - ignoring`

As the rack test driver does not support any option, we
get rid of all `default_set_options` before each system
spec that uses this driver.

spec/rails_helper.rb

@@ -47,7 +47,6 @@ end
 
 Capybara.exact = true
 Capybara.enable_aria_label = true
-Capybara.default_set_options = { clear: :backspace }
 Capybara.disable_animation = true
 
 OmniAuth.config.test_mode = true