Do not load not needed backend classes

[ihor-sviziev]

Description

This PR replaces the backend context with the frontend one. Since this controller is used on frontend, it doesn't make any sense to create classes needed for backend, e.g. backend session which is part of this context.

Basically, it just improves speed of this controller and reduces session locks on checkout

Manual Testing Steps

1.

Pre-Submission Checklist:

• [ ] You've updated the CHANGELOG following the steps here
• [ ] Internal Only - If this is a release, have you:
  • [ ] Updated the links in the CHANGELOG to point towards the new versions
  • [ ] Incremented the version in the following places: module.xml and composer.json

[cykolln]

This has been included in the latest release 4.0.7, thank you for another contribution @ihor-sviziev - much appreciated!