KAFKA-4698; VerifyError in kafka/client/ClientUtils due to -target:jvm-1.7

Author: Ismael Juma <ismael@juma.me.uk>

Reviewers: Jason Gustafson <jason@confluent.io>

Closes #2434 from ijuma/kafka-4698-target-1.7-verify-error

build.gradle

@@ -252,7 +252,6 @@ subprojects {
       "-deprecation",
       "-unchecked",
       "-encoding", "utf8",
-      "-target:jvm-${sourceCompatibility}".toString(),
       "-Xlog-reflective-calls",
       "-feature",
       "-language:postfixOps",