Revert "qdbusxml2cpp: Use the mtime on the input XML to avoid needless source changes."

This reverts commit 4d7f4b185e6aad6208c9add139db8404d063db64.
qdbusxml2cpp: Use the mtime on the input XML to avoid needless source changes.

These can cause rebuilds unnecessarily when repeatedly running qdbusxml2cpp.

Change-Id: I902954d4bed6fe68802183e51d82700fe30af437