Drop cryptoSafetyPrompt.properties line

The file cryptoSafetyPrompt.properties was deleted in tor-browser!965 (merged), and is no longer part of tor-browser branch in the translation repository, but tor-browser-build still try to copy the file in tor-browser-build. I imagine this is giving an error message during the build.

/cc @pierov can verify this and remove the line?

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