iOS: Remove assert when transferring or clearing first-responder

We'd transfer or clear first-responder in a lot more cases than just
when transferring to a new Qt window, such as when presenting a new
view-controller on top to send an e-mail or take a picture using the
camera.

Change-Id: I6b2a8a6d9fd99910b96a86cf9847b7ff0128f20a
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>