Remove duplicate assignment when initializing logical decoder context

The private data in the WAL reader is already getting set when
allocating it.

Author: Antonin Houska
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/30563.1555329094@localhost

src/backend/replication/logical/logical.c

@@ -178,8 +178,6 @@ StartupDecodingContext(List *output_plugin_options,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory")));
 
-	ctx->reader->private_data = ctx;
-
 	ctx->reorder = ReorderBufferAllocate();
 	ctx->snapshot_builder =
 		AllocateSnapshotBuilder(ctx->reorder, xmin_horizon, start_lsn,