Revert "gtype: Speed up type checks for final types"

Matthias Clasen requested to merge revert-final-type-check into main

This reverts commit 0ddea2d8.

The commit was based on the misunderstanding that types declared with G_DECLARE_FINAL_TYPE are actually non-derivable. But that is only the case for types defined with G_DEFINE_FINAL_TYPE.

Fixes: #2661 (closed)

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