Redundant initialization of table AM oid in relcache

Hi~

Just found 'relam' field is assigned twice during relcache initialization. We
have a specific area for initialization AM related stuff several lines later:

> /*
> * initialize the table am handler
> */
> relation->rd_rel->relam = HEAP_TABLE_AM_OID;
> relation->rd_tableam = GetHeapamTableAmRoutine();

so previous assignment seems not needed and can be removed.

—

Regards, Jingtang