do not obsolete old plugins in metapackage

poldek will pull in that metapackage for lost plugins:

...
  greedy upgrade claws-mail-plugin-trayicon-3.9.0-1.x86_64 to claws-mail-plugins-3.9.2-1.x86_64 (unresolved claws-mail = 3.9.0-1)
   claws-mail-plugin-trayicon-3.9.0-1.x86_64 obsoleted by claws-mail-plugins-3.9.2-1.x86_64