Remove parent type references

Remove parent type references

Requiring Constant, Enumeration, GeneratedType and TypeMember to
report its enclosing/parent type leads to circular dependencies, as
the parent type needs to report the objects as well. This leads to
the requirement to perform callouts from constructor -- utterly fusing
parents and children together.

As it turns out, though, these references are not used anywhere and
hence we can completely ditch them.

Change-Id: Ic9bfa6d0b69ffa3bdc1cd02c824405c9e22d0375
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>