build: Allow both OpenJDK and OracleJDK by default

Some users would like to build with Oracle JDK on Linux,
while others would like to build with OpenJDK on Darwin.

CM should build fine with either configuration, so drop
the OpenJDK checks to allow both of them.

Change-Id: I64d7887c2fab4dd301b07d7df0d19f28e97d80ab