Clean up some includes and comments in TAP test scripts

A few tests included File::Path::rmtree without using it, and a comment
related to the segment size for replication slot limits was wrong.

Author: Pavel Borisov, Bharath Rupireddy
Reviewed-by: Maxim Orlov
Discussion: https://postgr.es/m/CALj2ACU4-aNLX=DrUM8F7QDwynJKzYRiqOj_33NhnGbhDs5-kQ@mail.gmail.com

src/bin/pg_verifybackup/t/007_wal.pl

@@ -5,7 +5,6 @@
 
 use strict;
 use warnings;
-use File::Path qw(rmtree);
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 use Test::More;

src/test/recovery/t/004_timeline_switch.pl

@@ -4,7 +4,6 @@
 # Test for timeline switch
 use strict;
 use warnings;
-use File::Path qw(rmtree);
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 use Test::More;

src/test/recovery/t/019_replslot_limit.pl

@@ -9,8 +9,6 @@ use warnings;
 
 use PostgreSQL::Test::Utils;
 use PostgreSQL::Test::Cluster;
-
-use File::Path qw(rmtree);
 use Test::More;
 use Time::HiRes qw(usleep);
 
@@ -429,7 +427,7 @@ sub advance_wal
 {
 	my ($node, $n) = @_;
 
-	# Advance by $n segments (= (16 * $n) MB) on primary
+	# Advance by $n segments (= (wal_segment_size * $n) bytes) on primary.
 	for (my $i = 0; $i < $n; $i++)
 	{
 		$node->safe_psql('postgres',