PySHiELD: Removing --no_legacy_namelist flag

[jjuyeonkim]

Description
This is a follow-up (clean-up) from PR NOAA-GFDL/NDSL#297.

It removes the flag --no_legacy_namelist flag from the github workflows.

How Has This Been Tested?
It passes CI

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• Targeted model if this changed was triggered by a model need/shortcoming

[romanc]

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