QbsProjectManager: Fix "build single file" use case.

The approach used in qbs was broken conceptually after the recent
rewrite of the build logic. Use the new facility.

Change-Id: I219f0ae3ec4ea86304896fc1671dbdc97028a7e4
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
QbsProjectManager: Remove invalid QTC_ASSERT().

The asserted condition is no longer required to be true.

Change-Id: I840ef5670d87c4372d4270623aa236833a8a3fda
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>