migrating to usage of rbe_preconfig and remove bazel-toolchains (#134)
migrating to usage of rbe_preconfig and remove bazel-toolchains (#134)

* update bazel-toolchain 5.1.2

* remove deprecated rbe_autoconfig

* use rbe_preconfig and remove usage of bazel-toolchains as it was only included for the deprecated rbe_autoconfig

* change name to buildkite_config

* reformat WORKSPACE
bazelbuild/rules_proto,bazelbuild/rules_proto