BuildManager: Don't count disabled steps in progress

QFuture limits the signals that are emitted for progress. A disabled
step takes so little time that its progress information is lost.
Since technically a disabled step never finishes, counting them in the
progress was dubious anyway.

Task-number: QTCREATORBUG-10606
Change-Id: I1860c65242282e8e2890f4977c487a6e6dcfbf2d
Reviewed-by: Daniel Teske <daniel.teske@digia.com>