Remove the rustc submodule from bootstrap.sh

The first download of Redox sources take so long because the rustc submodule fetch the LLVM sources, which are heavy and the deltas processing can take some time in a low-end computer.

As the rustc submodule will just be used by skilled developers, it can be removed.

Similar request - #1339

Update: Jeremy said that the rustc submodule should be moved to the prefix folder.