Move the NCBI field map to config

[j23414]

Description of proposed changes

To be consistent with the handling of nextclade-field-map.tsv, move ncbi-field-map.tsv to config. This change is motivated by the following comment:

• https://github.com/nextstrain/dengue/pull/13#discussion_r1373370410

Related issue(s)

• https://github.com/nextstrain/dengue/pull/13#discussion_r1373370410

Checklist

• [x] Checks pass