Deprecate unnecessary NameSpace methods

[whiteknight]

I have removed a few unnecessary method calls in src/oo.c that were eating up significant performance (coretest time on my system dropped from ~4:40 to ~3:40 by removing them). Now, I think the following methods on NameSpace are unused and should probably be deprecated/removed:

• set_class
• get_associated_methods
• get_associated_vtable_methods

Removing these methods also helps work towards decoupling Class and NameSpace PMC types, which is a goal several devs have. These methods are typically for internal use and are not typically used by users of Parrot directly, so I suspect we do not need to offer an alternative to any of these things