Doc: remove obsolete doc build variables

The HELP_DEP_FILES variable is not used by QDoc for Qt 5.
Also changed the app name to IFW. It seems only to be
used for the help file.

Change-Id: Ib8e1ceeacf55c2aa5f71b67446de0ceb8586a2f6
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>