bintree: don't provide default VERSION header

We want to make sure that we know when the VERSION header is missing,
so we shouldn't provide a default value.
diff --git a/pym/portage/dbapi/bintree.py b/pym/portage/dbapi/bintree.py
index 36bf735..ebec11f 100644
--- a/pym/portage/dbapi/bintree.py
+++ b/pym/portage/dbapi/bintree.py
@@ -273,7 +273,6 @@
 
 			# Populate the header with appropriate defaults.
 			self._pkgindex_default_header_data = {
-				"VERSION"      : str(self._pkgindex_version),
 				"CHOST"        : self.settings.get("CHOST", ""),
 				"repository"   : "",
 			}