Remove redundant memory context switches in BeginCopyFrom().

This is probably a leftover from code refactoring.

Japin Li

Discussion: https://postgr.es/m/MEYP282MB16693DDABDFEC7949AC31857B6599@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

Author: tglsfdc

src/backend/commands/copyfrom.c

@@ -1340,10 +1340,6 @@ BeginCopyFrom(ParseState *pstate,
 
 	cstate->whereClause = whereClause;
 
-	MemoryContextSwitchTo(oldcontext);
-
-	oldcontext = MemoryContextSwitchTo(cstate->copycontext);
-
 	/* Initialize state variables */
 	cstate->eol_type = EOL_UNKNOWN;
 	cstate->cur_relname = RelationGetRelationName(cstate->rel);