Remove the OpenJDK submodule

[qinsoon]

We should adopt the pattern in https://github.com/mmtk/mmtk-v8, which does not have the v8 repo as a part of the binding repo. The submodule is considered as bad, as it implies that the VM repo is part of the binding, which is logically incorrect. Basically we would need to achieve this:

• The binding does not include any submodule.
• The binding still refers to a version/commit hash of the VM repo
• In the CI tests for the binding, we use the version/commit hash to test.