Code review: 8046192 Eliminate SNMP dependencies to the internal APIs from open jdk modules

[shanliang]

Hi,

The fix is to remove unnecessary exports for jdk.snmp module.

bug: https://bugs.openjdk.java.net/browse/JDK-8046192
webrev: http://cr.openjdk.java.net/~sjiang/JDK-8046192/00/

Thanks,
Shanliang