Remove mattermost failure webhook

Description

Resolves this issue

Risk

Rollback Plan

Testing

File is not referenced anywhere else in the pipeline code, so removing it shouldn't have any negative impacts. However, I did test 2 pipelines in Zelda:

• https://code-ib-zelda.staging.dso.mil/dsop/MJ/ubi8-minimal/-/pipelines/54095
• https://code-ib-zelda.staging.dso.mil/dsop/MJ/ubi9-micro/-/pipelines/54096