Doc: don't use the term SDK in connection with Qt

Replace with "Qt distributions" or "installers" or rewrite
in some other way.

Remove references to installing Qt 4 SDK, because it is
no longer available for downloading.

Change-Id: I291f82702e21d953f2efbb0e634d6454b4bc17b9
Reviewed-by: hjk <hjk121@nokiamail.com>