Remove transport-netty3-client mention (#21650)

Now netty3 is gone, this mention must also be removed.

build.gradle

@@ -112,7 +112,6 @@ subprojects {
     "org.elasticsearch.distribution.deb:elasticsearch:${version}": ':distribution:deb',
     "org.elasticsearch.test:logger-usage:${version}": ':test:logger-usage',
     // for transport client
-    "org.elasticsearch.plugin:transport-netty3-client:${version}": ':modules:transport-netty3',
     "org.elasticsearch.plugin:transport-netty4-client:${version}": ':modules:transport-netty4',
     "org.elasticsearch.plugin:reindex-client:${version}": ':modules:reindex',
     "org.elasticsearch.plugin:lang-mustache-client:${version}": ':modules:lang-mustache',