[CI] Delete unused `PR Differences Mutants` workflow

[fdefelici]

We've decided to delete this workflow after discussing it with @wileyj.
While the original idea was good, in practice it proved unworkable:

• In CI environments, the workflow often timed out or failed, even after running for several hours.
• On dedicated hardware (24-core system), a full mutant test could take up to a week to complete.

Given these performance and reliability concerns, we concluded that maintaining this workflow is not practical. So we delete it by now to avoid maintainance issue (such as PAT removal, and so on...)

[wileyj]

tagging @ASuciuX for any comments as original author