Remove build status indicator from README

The image link is broken and reaches out to
Jenkins which is getting deprecated anyway.
Removing the indicator.

README.md

-[![Build Status](https://grpc-testing.appspot.com/job/gRPC_master/badge/icon)](https://grpc-testing.appspot.com/job/gRPC_master)
-
 [gRPC - An RPC library and framework](http://github.com/grpc/grpc)
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