Don't write QTDIR to the *.vcxproj.user file

We are using MSBuild property sheets / environment variables to set the proper QTDIR directory for every project on every developer workstation.

This works all well and fine until an active code workspace is moved to another machine (e.g. when code resides on a external hard drive), because the Qt plugin ends up writing a copy of QTDIR into the *.vcxproj.user files, and of course they don't get updated once the "actual" QTDIR changes because the project files have been moved around.

To me it seems like Qt would just fine without this entry, as QTDIR is already set anyhow.

Removing this entry or at least adding an option to not write this entry would make our life a bit easier, because we wouldn't have to delete each and every *.user file from the entire source tree after moving it to another machine.