benchmarks: Remove -javaagent in CreateStartScripts
benchmarks: Remove -javaagent in CreateStartScripts

The Jetty ALPN is not needed for benchmarks. This change removes the
-javaagent lines from the benchmarks/build.gradle file as suggested by
@ejona86.

The shell script references the Jetty ALPN agent stored in ~/.gradle which
is not available when just using the application artifacts. interop-testing
has a hack to make the agent work, but we don't really care about "plain"
Java performance for benchmarks so it is easier to just remove it.
dapengzhang0/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,ejona86/grpc-java,ejona86/grpc-java,ejona86/grpc-java,ejona86/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java