Debugger: Revert parts of the 'arrayIndex' optimization

The parent's address is not always the base address of the array
data, but often a wrapper like QVector, so the correct computation
of the item's address is more involved, removing the benefits of
the optimization.

Change-Id: Iecb19799addc1502649fefbad0953b77947f4193
Reviewed-by: Christian Stenger <christian.stenger@qt.io>