Doc: Remove Ubuntu 14.04, Ubuntu Touch from Supported Platforms

Ubuntu 14.04 (64-bit) is no longer part of the CI. Also remove the
last mention of Ubuntu Touch.

Change-Id: I6bd480aa4a18c882f5528d3fd3c43aed0a1e4ceb
Reviewed-by: Heikki Halmet <heikki.halmet@qt.io>