Replace assign-colors.py

[joverlee521]

There is a shared assign-colors available now that can replace the custom assign-colors.py in various pathogen repos in both phylogenetic and nextclade workflows.

Pathogens to update based on GH search:

• avian-flu
• chikv
• dengue
• hbv
• hmpv
• lassa: done as part of Add standardized multiple inputs support lassa#77
• mpox
• mumps
• ncov: keeping as-is since we are not adding vendored scripts to this repo
• norovirus: done as part of Add standardized multiple input support norovirus#34
• oropouche
• rsv
• rubella
• yellow-fever