Revert "postgres provider: quote compound key columns in uri"

This reverts commit daa6510.

I've tested that reverting this does not re-introduce bug
http://hub.qgis.org/issues/13710

src/providers/postgres/qgspostgresprovider.cpp

@@ -248,7 +248,7 @@ QgsPostgresProvider::QgsPostgresProvider( QString const & uri )
       QString delim;
       Q_FOREACH ( int idx, mPrimaryKeyAttrs )
       {
-        key += delim + quotedIdentifier( mAttributeFields.at( idx ).name() );
+        key += delim + mAttributeFields.at( idx ).name();
         delim = ',';
       }
     }