VPN landing page clean up

[wen-2018]

#16789 (review)

Following the VPN landing page reversion to variant a, there are a few more clean ups we could do:

• media/css/products/vpn/bundle-promo.scss this stylesheet is specific to the bundle promo sections, can be removed now
• media/js/products/vpn/promo-tabs.js this file can be removed since the promo tabs are no longer needed
• images used specifically for variant c in this dir media/img/products/vpn/landing-refresh can be removed
• we have updated some helper functions in bedrock/products/templatetags/misc.py to include bundle_monitor_relay. They should be updated as well as the updated tests in bedrock/products/tests/test_helper_misc.py