Build: Remove extra copies of netty license (#23361)

The dependencyLicenses check has the ability to map multiple jar files
to the same license file. However, netty was not taking advantage of
this, and had duplicate copies of its license/notice files for each jar.
This commit reduces the copies to one and uses the mapping feature.

Author: rjernst

modules/transport-netty4/build.gradle

@@ -42,11 +42,8 @@ dependencies {
     compile "io.netty:netty-transport:4.1.7.Final"
 }
 
-integTest {
-    includePackaged = true
-}
-integTestCluster {
-    numNodes = 2
+dependencyLicenses {
+  mapping from: /netty-.*/, to: 'netty'
 }
 
 thirdPartyAudit.excludes = [