Fix tst_Dialogs::authenticationDialogRequested

Chromium returns unexpected proxy reply if localhost is used.

Since proxy resolver works now, the workaround to trigger dialog
with "localhost proxy" can be skipped.

Change-Id: I26d88c43779809e4f481101266e6b8784a6b196d
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>