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ARROW-3394: [Java] Remove duplicate dependency in Flight for grpc-netty

This removes the duplicate entry of grpc-netty in the Flight pom, which leads to the following warning:

```
[WARNING] Some problems were encountered while building the effective model for org.apache.arrow:arrow-flight:jar:0.11.0-SNAPSHOT
[WARNING] 'dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique: io.grpc:grpc-netty:jar -> duplicate declaration of version ${dep.grpc.version} @ org.apache.arrow:arrow-flight:[unknown-version], /home/bryan/git/arrow/java/flight/pom.xml, line 55, column 17
```

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #2679 from BryanCutler/java-remove-dup-flight-dep-ARROW-3394 and squashes the following commits:

453483e5 <Bryan Cutler> remove duplicate dep for grpc-netty