Remove redundant assignments in copyfrom.c

The tuple descriptor and the number of attributes are assigned twice to
the same values in BeginCopyFrom(), for what looks like a small thinko
coming from the refactoring done in c532d15ddd.

Author: Jingtang Zhang
Discussion: https://postgr.es/m/CAPsk3_CrYeXUVHEiaWAYxY9BKiGvGT3AoXo_+Jm0xP_s_VmXCA@mail.gmail.com

src/backend/commands/copyfrom.c

@@ -1533,8 +1533,6 @@ BeginCopyFrom(ParseState *pstate,
 		cstate->rteperminfos = pstate->p_rteperminfos;
 	}
 
-	tupDesc = RelationGetDescr(cstate->rel);
-	num_phys_attrs = tupDesc->natts;
 	num_defaults = 0;
 	volatile_defexprs = false;