Doc: remove note about Symbian support

Nokia has discontinued support for Qt development for Symbian
devices. The Nokia Qt SDK 1.x.x is no longer available for
download at Nokia sites.

Change-Id: Iec7e316729c6ffe2e698070f4833330e5a2d95da
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>