spec: drop the old %beta hack about %__os_install_post
    
    This always was broken and the %__os_install_post macro was
    redefined also for non-beta builds;  that's because
    %{?x:%global y z} defines %y even when %x is zero.
    
    If we really wanted this %beta/%post hack, it would have to look
    similarly to:
    %{lua:if rpm.expand("%beta") ~= "0" then rpm.define('...') end}
    On the other hand, I'm not sure that this is actually needed
    nowadays so I'm removing it completely.  Please let us know if you
    happen to need the %__os_install_post redefinition.
    
    Originally added by commit 9636c11b0d6049d23.
    
    Version: 9.6.3-5