Remove support for kaniko and buildkit backend

[dekiel]

Description

Remove support for building images using kaniko and buildkit backend from image-builder tool.

Reasons

Image-builder is using a ADO backend as SLC-29 certified build system.

Acceptance Criteria

• [ ] Remove code for kaniko backend.
• [ ] Remove code for buildkit backend.
• [ ] Remove prowjob building buildkit image-builder.
• [ ] Update names and paths to not use kaniko name for image-builder tool with ado backend.
• [ ] Make ADO backend only supported build backend.
• [ ] Remove flags to enable ADO backend.