make CMOP::Immutable::Trait stop lying about its metaclass

make CMOP::Immutable::Trait stop lying about its metaclass

Class::MOP::Class is an immutable class, so its metaclass really does
need to be the immutable version of itself
(Class::MOP::Class::Immutable::Class::MOP::Class). Hiding this just
causes confusing issues. Class::MOP::Class::Immutable::Class::MOP::Class
isa Class::MOP::Class, so this shouldn't actually affect the behavior of
any code (that wasn't doing things wrong to begin with).