Remove JDK 9 flag on CI builds

The special JDK 9 flags are no longer needed after the JDK maintainers
decided at the last minute to permit illegal access by default.

Relates elastic/x-pack-elasticsearch#3250

Original commit: elastic/x-pack-elasticsearch@a4cfcd8d1b

dev-tools/ci

@@ -67,16 +67,6 @@ case $key in
             "-Dtests.jvm.argline=-Dbuild.snapshot=false"
         )
         ;;
-    jdk9)
-        GRADLE_CLI_ARGS=(
-            "-Pxpack.kibana.build=false"
-            "--info"
-            "check"
-            "-Dtests.network=true"
-            "-Dtests.badapples=true"
-            -Dtests.jvm.argline="--add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.nio=ALL-UNNAMED --add-opens=java.base/java.nio.file=ALL-UNNAMED --add-opens=java.base/java.security.cert=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.base/javax.net.ssl=ALL-UNNAMED"
-        )
-        ;;
     *)
         echo "Unsupported cli argument $1. Allowed arguments are packagingTest or check. No argument defaults to check."
         exit 1;;