Revert "Do not build developer documentation for qt creator"

Actually the -dev.qch should be build (albeit it isn't 
registered automatically). Anyhow, the index.html conflict
the change was supposed to fix isn't caused by the -dev.qch
file being built, but by a mixup of .qdoc files for the
qtcreator.qch one.

This reverts commit bd56fbd85115ee19d9e4ed394445f2ba12dd45a6

Change-Id: I31ee0307a06394a4faa2532e13f1f5433567a5b9
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>