Remove no-longer-needed compatibility hack

Our Perl version requirement was raised to 5.14 by commit 4c1532763a

Discussion: https://postgr.es/m/20221017081649.fjcd2kjqif77uyf2@alvherre.pgsql

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -2933,10 +2933,7 @@ sub corrupt_page_checksum
 package PostgreSQL::Test::Cluster::V_11
   ;    ## no critic (ProhibitMultiplePackages)
 
-# parent.pm is not present in all perl versions before 5.10.1, so instead
-# do directly what it would do for this:
-# use parent -norequire, qw(PostgreSQL::Test::Cluster);
-push @PostgreSQL::Test::Cluster::V_11::ISA, 'PostgreSQL::Test::Cluster';
+use parent -norequire, qw(PostgreSQL::Test::Cluster);
 
 # https://www.postgresql.org/docs/11/release-11.html
 
@@ -2964,8 +2961,7 @@ sub init
 package PostgreSQL::Test::Cluster::V_10
   ;    ## no critic (ProhibitMultiplePackages)
 
-# use parent -norequire, qw(PostgreSQL::Test::Cluster::V_11);
-push @PostgreSQL::Test::Cluster::V_10::ISA, 'PostgreSQL::Test::Cluster::V_11';
+use parent -norequire, qw(PostgreSQL::Test::Cluster::V_11);
 
 # https://www.postgresql.org/docs/10/release-10.html