Revert "Safely finalize and destroy DebugSession objects"

This fix broke the ABI, kdev-python no longer compiles. Therefore it must be reverted in the stable 5.6 branch.

This reverts commit 37ff587e.

See the comment thread at !161 (comment 120667).

After I commit this into 5.6, I'll merge 5.6 into master and revert the "Revert" commit in master. I found that kdev-python, kdev-xdebug, kdev-crossfire have classes that override IDebugSession via Github search. I am going to try and adapt these plugins to the API change in the reverted commit, which will be reapplied to master. But I don't see kdev-crossfire at https://invent.kde.org/kdevelop. Is it no longer maintained? Or should it still be fixed somewhere else?

Will also have to set Version Fixed In for Bug 425991 to 5.7.0.