Remove CONFIG+=declarative_debug from imported build line

Prevent CONFIG+=declarative_debug to show up in "custom arguments"
of an imported build.

Change-Id: Ib598addcb31b65853eea430a086f614c6dd498fd
Reviewed-on: http://codereview.qt.nokia.com/4044


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>