remove maxdiffusion_gpu_e2e DAG

[michelle-yooh]

Description

MaxDiffusion GPU build is not actively maintained anymore. This PR removes maxdiffusion_gpu_e2e DAG. The obsolete GPU image constant is removed too.

Tests

Please describe the tests that you ran on Cloud VM to verify changes.

Instruction and/or command lines to reproduce your tests: N/A

List links for your tests (use go/shortn-gen for any internal link): N/A

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run one-shot tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.