Remove diag table schema

[J-Lentz]

Description
Remove the diag table schema, as it has been moved to the gfdl_msd_schemas repository. Add references to the schema and the new repository to the diag manager's README.md and diag_yaml_format.md files.

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
• make distcheck passes

[uramirez8707]

Thanks! Looks good to me