Removal of Unsafe::defineAnonymousClass left a dangling C++ class

The class CPSlot is no longer of service in constantPool.hpp, since the only possible contents of a CPSlot is a Symbol pointer. The constant "_pseudo_bit" is a dead leftover from the days of CP patching.

I suggest removing CPSlot altogether.

This appears to be an unintentional leftover from JDK-8243287, since there does not seem to be any email discussion of the status of CPSlot.