travis: don't reinstall rustup

travis now installs rust using rustup

.travis.yml

@@ -34,8 +34,6 @@ before_install:
   - test "$TRAVIS_OS_NAME" = "osx" || docker run --rm --privileged multiarch/qemu-user-static:register
 
 install:
-  - curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain $TRAVIS_RUST_VERSION
-  - source ~/.cargo/env
   - case $TARGET in
       x86_64-apple-darwin | x86_64-unknown-linux-gnu) ;;
       thumbv*eabi*) rustup component add rust-src ;;