remove deprecated method to avoid warning

[hxingpax]

fix warning of cargo test:

warning: use of deprecated method `cmake::Config::uses_cxx11`: no longer does anything, C++ is determined based on `cc::Build`, and the macOS issue has been fixed upstream
   --> grpc-sys/build.rs:255:16
    |
255 |         config.uses_cxx11().build()
    |                ^^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

[ti-chi-bot[bot]]

Welcome @hxingpax! It looks like this is your first PR to tikv/grpc-rs 🎉

[BusyJay]

Hi, failed CI is not related to your changes, but can you also fix them in this PR?

[taddes]

I'll chime in since @BusyJay and I discus this issue in #654 and related context to cc-rs and cmake. I'd suggest removing this from the documentation comment and example in cross_compile.md under All diff in fn build_grpc

[BusyJay]

Friendly ping.

Hi, @hxingpax do you still have time to work on this?

[taddes]

Hey @BusyJay , I'm finally getting to this and have my PR, however the link to the docs on setting up DCO commits is dead in the ti-chi-bot message, as there's no Contributing doc. My commits are GPG signed, as we have that policy over at Mozilla.