doc: remove template comments in old release notes

doc: remove template comments in old release notes

The release notes comments mention how to generate the documentation
with the old & removed build system.

Rather than fixing these comments, all old release notes comments
are removed, because they are useful only for the current release.

Few extra blank lines are removed for consistency.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: John McNamara <john.mcnamara@intel.com>