Since g-i 1.40.0 we are using the .lib file(s) to determine the DLL(s) that
the introspection files should look for, instead of directly using the
DLL(s) directly, so it is no longer necessary to make copies of the GDK
.lib file to match the <namespace-name>-<namespace-version> convention when
we are building the introspection files for GdkWin32.  Remove that from the
introspection build process as a result.