Remove unused variable in recovery/t/006_logical_decoding.pl.

Author: Daniil Davydov <3danissimo@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAJDiXggmZWew8+SY_9o0atpmaJmPTL25wdz07MrDoqCkp4D1ug@mail.gmail.com

src/test/recovery/t/006_logical_decoding.pl

@@ -21,7 +21,6 @@ $node_primary->append_conf(
 wal_level = logical
 ));
 $node_primary->start;
-my $backup_name = 'primary_backup';
 
 $node_primary->safe_psql('postgres',
 	qq[CREATE TABLE decoding_test(x integer, y text);]);