(SUP-4210) Remove logic for EOL Postgres versions

This commit cleans up logic in the `psql_metrics` script that supported
Postgres versions older than 10.

These versions are all end of life upstream. Puppet Enterprise upgraded
to PostgreSQL 11 with the 2019.2 release and Open Source PuppetDB started
requiring PostgreSQL 11 with the 7.0.0 release.

Author: Sharpie

files/psql_metrics

@@ -244,8 +244,6 @@ module PuppetMetricsCollector
       else
         @pg_version = Gem::Version.new(@pg_version.strip)
       end
-      # Some functions and statistics views were re-named in Postgres 10.0.
-      @is_pg10 = Gem::Requirement.new('>= 10.0').satisfied_by?(@pg_version)
 
       add_data!(@result, :checkpoints, sql_query(<<-EOS))
 SELECT json_build_object(
@@ -314,16 +312,14 @@ ORDER BY
 LIMIT 1;
 EOS
 
-      lsn_diff = @is_pg10 ? 'pg_wal_lsn_diff' : 'pg_xlog_location_diff'
-      current_lsn = @is_pg10 ? 'pg_current_wal_lsn()' : 'pg_current_xlog_location()'
       add_data!(@result, :replication_slots, sql_query(<<-EOS))
 SELECT json_object_agg(
   slot_name,
   json_build_object(
     'active', active,
     'xmin', xmin,
     'catalog_xmin', catalog_xmin,
-    'lag_bytes', #{lsn_diff}(#{current_lsn}, restart_lsn)
+    'lag_bytes', pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)
   )
 )
 FROM