[FIX] l10n_ch: remove obsolete po file

Description of the issue/feature this MR addresses:

Removing obsolete po files in l10n_ch

Current behavior before MR:

Updating l10n_ch creates error: psycopg2.ProgrammingError: ON CONFLICT DO UPDATE command cannot affect row a second time

Two identical po files in l10n_ch/i18n_extra

Desired behavior after MR is merged:

No more update error