Revert "stack: clear accessible parent when removing child"

Revert "stack: clear accessible parent when removing child"

This reverts commit 40d4441fd85742837d85d8509dc89355d39a1cdc.

The accessible parent of the child widget in a GtkStackPage is cleared
when the GtkATContext gets disposed, so we don't need to unset it
ourselves. This also avoids a temporary vivification of the GtkATContext
during dispose.