In Monero 12.0 the version code got rewritten and, as a result, the
variable substitution export attribute no longer applies (since
`version.cmake` got moved and renamed to `Version.cmake`), but this bit
of cruft to rewrite the release tag to "-release" and the attribute are
still there, but cannot ever be invoked.
Remove them. I specifically don't want to fix it because source exports
can come from anywhere, *not* only tagged versions, so putting
"-release" in the version string could be highly misleading.