Remove env files support in the image builder

[Sawthis]

Description

Remove env files support in the image builder.

Reasons

Better solution without using env files was found.

Acceptance Criteria

[ ] check if someone is using env files e.g third-party-images, if yes migrate it to the solution with computing tags and dockerfiles before calling image builder [ ] remove support for env files in the code [ ] remove documentation connected with env files

Attachments

Support for env files was introduced in https://github.com/kyma-project/test-infra/issues/10413