clean the pot file

By removing trademark translations (evernote, yinxiang, zsync) as well as words that don't need to be translated ( sandbox, << and >>)
The `zsync` plugin of koreader has nothing to do with the zsync binary used in the OTA update routine. I didn't know the existence of zsync binary when I wrote the plugin and definitely it's not a trademark. And it has already been translated into Chinese as "无线互传"(wireless transporter, but this phase is too long to be a plugin name in English). I suggest we rename the plugin and make it translatable.