Remove sr-ec from latin l10n list

Summary:
The sr-ec code matches Serbian written in Cyrillic.

Fixes T1238.

Test Plan: jsonlint l10n/latin-whitelist.json

Reviewers: Thibaut120094

Reviewed By: Thibaut120094

Maniphest Tasks: T1238

Differential Revision: https://devcentral.nasqueron.org/D1022