i18n: zh_CN: fix 'duplicate message definition'

Error reported by Weblate:

msgmerge --no-wrap --backup=none --previous --update /home/www-data/weblate-hosted/vcs/kallithea/kallithea/kallithea/i18n/zh_CN/LC_MESSAGES/kallithea.po /home/www-data/weblate-hosted/vcs/kallithea/kallithea/kallithea/i18n/kallithea.pot

/home/www-data/weblate-hosted/vcs/kallithea/kallithea/kallithea/i18n/zh_CN/LC_MESSAGES/kallithea.po:6927: duplicate message definition...
/home/www-data/weblate-hosted/vcs/kallithea/kallithea/kallithea/i18n/zh_CN/LC_MESSAGES/kallithea.po:9: ...this is the location of the first definition
msgmerge: found 1 fatal error


Problem is introduced with commit 880e2f07651b3eaf88d01e71ac7f3cbf88df163c.