Remove dependency on boringssl-src if secure is not activated

[Hugues de Valon]

Hello 👋

In the README, it is written:

secure feature enables support for TLS encryption and some authentication mechanism. When you do not need it, for example when working in intranet, you can disable it by using the following configuration

However, even if it is disabled, boringssl-src will be included in the dependencies via grpcio-sys (see here).

Removing the need of compiling boringssl-src when the secure feature is disabled will make compilation faster and also easier for cross-compilation targets 👍

I am happy to have a go at it and try to modify the build.rs file to something that'd work.

[Jay]

Sure, feel free to send us your patch when ready!