Revert the change to the ref counting of initial wrappers of custom subclasses

This reverts the commit "Ensure timely destruction of initial wrapper of custom
subclasses", eb6ff1b1.

It turns out that the change causes issues with subclasses inheriting from
GInitiallyUnowned.  Instances of classes derived from GInitiallyUnowned would
be destroyed prematurely when code in INIT_INSTANCE manages to sink the
initial, floating reference.

Reported by Kevin Ryde.