msi: Remove an unused declaration

[Andrew Talbot]

Changelog:
    msi: Remove an unused declaration.

diff --git a/dlls/msi/msipriv.h b/dlls/msi/msipriv.h
index f856e66..b72e1d7 100644
--- a/dlls/msi/msipriv.h
+++ b/dlls/msi/msipriv.h
@@ -1040,7 +1040,6 @@ extern void ui_actiondata(MSIPACKAGE *, LPCWSTR, MSIRECORD *);
 /* string consts use a number of places  and defined in helpers.c*/
 extern const WCHAR cszSourceDir[];
 extern const WCHAR cszSOURCEDIR[];
-extern const WCHAR szProductCode[];
 extern const WCHAR cszRootDrive[];
 extern const WCHAR cszbs[];