remove Windows MSI dependency on pbspro.spec so that pbspro.spec need not be maintained in the repo
As of now, Windows MSI depends on pbspro.spec file for getting the pbs version (as can be seen here) and that is the only reason behind maintaining pbspro.spec in our repo, even though we have pbspro.spec.in.
Due to this, whenever, we change pbspro.spec.in, we also need to change pbspro.spec which is unnecessary.
We need to provide MSI a Windows native way to generate pbs version and remove pbspro.spec from the repo.
MSI should use something that is native to Windows instead of pbspro.spec which essentially is a "Linux" file.