Remove reference to Billing::Integrations

[pi3r]

This is a remnant of when all offsites gateway were extracted to
OffsitPayments. The namespace Billing::Integrations doesn't
exist anymore.

References #3503 (comment)

irb(main):001:0> require 'activemerchant'
=> true
irb(main):002:0> ActiveMerchant
=> ActiveMerchant
irb(main):003:0> ActiveMerchant::Billing
=> ActiveMerchant::Billing
irb(main):004:0> ActiveMerchant::Billing::Integrations
Traceback (most recent call last):
        4: from /opt/rubies/2.6.5/bin/irb:23:in `<main>'
        3: from /opt/rubies/2.6.5/bin/irb:23:in `load'
        2: from /opt/rubies/2.6.5/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        1: from (irb):4
NameError (uninitialized constant ActiveMerchant::Billing::Integrations)

[paracycle]

👍