Delete qml_plugin_import.cpp file only during distclean

For static builds of Qt Quick apps, qmake generates a qml_plugin_import.cpp
file. Just like the Makefiles, it should be removed only for distclean,
not in the clean step. This is what we do for non-qml plugins, too.

Change-Id: I5a3f2e7d27c3ffd5161162a8a03e4dd9c9245af5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>