Remove redundant relam initialization

This struct member is initialized again a few lines below in the same
function.  This is cosmetic, so no backpatch.

Reported-by: Jingtang Zhang <mrdrivingduck@gmail.com>
Discussion: https://postgr.es/m/AFF74506-B925-46BB-B875-CF5A946170EB@gmail.com

src/backend/utils/cache/relcache.c

@@ -1939,7 +1939,6 @@ formrdesc(const char *relationName, Oid relationReltype,
 	relation->rd_rel->relallvisible = 0;
 	relation->rd_rel->relkind = RELKIND_RELATION;
 	relation->rd_rel->relnatts = (int16) natts;
-	relation->rd_rel->relam = HEAP_TABLE_AM_OID;
 
 	/*
 	 * initialize attribute tuple form