unbreak static qml imports again

a) qmlimportscanner has no built-in -importPath, so it can't be omitted
even for non-prefix builds, and b) the QMLPATHS variable is also used
further down, so we can't just do away with it.

amends a658fa40.

Change-Id: I42a47a82fe13694fbac3c4a3962ebbe1d7e7865b
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>