Fix tst_QAccessiblity failure.

This started to fail after 34f1758428282a327c12b0d8040061c1f67ecc7f.
Or actually, the test crashes on my machine. Reason is that the the test
first creates a top-level line edit (which then gets its own backing
store). The line edit is then reparented into another top-level. When
the line edit is destroyed extra->topextra->backingStore is true and we
delete the backing store it first got when created as a top-level.
However, the line edit was reparented so this backing store is not the
"active" one. We should still delete topextra->backingstore, but we must
also remove any pointer references to the line edit in the "active" backing
store.

Reviewed-by: jbarron