Disable enqueuing of Open Sans and other fonts used by WordPress from Google.
This plugin stops loading of Open Sans and other fonts used by WordPress and bundled themes (Twenty Twelve, Twenty Thirteen, Twenty Fourteen, Twenty Fifteen) from Google Fonts.
Reasons for not using Google Fonts might be privacy and security, local development or production, blocking of Google's servers, characters not supported by font, performance.
Disable Google Fonts is a very lightweight, it has no settings, just activate it and it works immediately.
And it's on GitHub.