software/postgresql: Don't build postgresql twice

component/postgresql/buildout.cfg defines both postgresql92 and
postgresql sections, with postgresql being just

    [postgresql]
    <= postgresql92

On the other hand, software/postgresql uses postgresql92 in parts, but
bin = ${postgresql:location}/bin in instance, which leads to _both_
postgresql and postgresql92 being built, and postgresql92 not used at all.

Let's fix it and use postgresql everywhere.

/cc @kazuhiko

software/postgres/software.cfg

@@ -7,7 +7,7 @@ extends =
 parts =
   slapos-cookbook
   instance
-  postgresql92
+  postgresql
 
 
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