Remove useless Bundler config
Remove useless Bundler config

This configuration setting isn't available in newer versions of Bundler,
and is wiped from this config on `bundle install`.

solidusio/solidus_paypal_braintree,solidusio/solidus_paypal_braintree,solidusio/solidus_paypal_braintree,solidusio/solidus_paypal_braintree