Debugger: Disable QModelIndex dumper

The inferior calls to rowCount() and columnCount() used to create
data() display are troublesome when debugging an actual
issue with item models and not too helpful otherwise. People needing
that information can still evaluate idx.data() etc manually.

Change-Id: If736afbf8fc44cf28e7f07e5f2c3b039a2f4984c
Reviewed-by: Christian Stenger <christian.stenger@qt.io>