Remove check supports_drop_index_concurrently?

It's not needed anymore as we require Postgres >= 9.6 now

db/migrate/20180215181245_users_name_lower_index.rb

@@ -20,10 +20,6 @@ class UsersNameLowerIndex < ActiveRecord::Migration[4.2]
   def down
     return unless Gitlab::Database.postgresql?
 
-    if supports_drop_index_concurrently?
     execute "DROP INDEX CONCURRENTLY IF EXISTS #{INDEX_NAME}"
-    else
-      execute "DROP INDEX IF EXISTS #{INDEX_NAME}"
-    end
   end
 end

db/migrate/20180504195842_project_name_lower_index.rb

@@ -22,11 +22,7 @@ class ProjectNameLowerIndex < ActiveRecord::Migration[4.2]
     return unless Gitlab::Database.postgresql?
 
     disable_statement_timeout do
-      if supports_drop_index_concurrently?
       execute "DROP INDEX CONCURRENTLY IF EXISTS #{INDEX_NAME}"
-      else
-        execute "DROP INDEX IF EXISTS #{INDEX_NAME}"
-      end
     end
   end
 end

lib/gitlab/database/migration_helpers.rb

@@ -108,9 +108,7 @@ module Gitlab
             'in the body of your migration class'
         end
 
-        if supports_drop_index_concurrently?
         options = options.merge({ algorithm: :concurrently })
-        end
 
         unless index_exists?(table_name, column_name, options)
           Rails.logger.warn "Index not removed because it does not exist (this may be due to an aborted migration or similar): table_name: #{table_name}, column_name: #{column_name}" # rubocop:disable Gitlab/RailsLogger
@@ -136,9 +134,7 @@ module Gitlab
             'in the body of your migration class'
         end
 
-        if supports_drop_index_concurrently?
         options = options.merge({ algorithm: :concurrently })
-        end
 
         unless index_exists_by_name?(table_name, index_name)
           Rails.logger.warn "Index not removed because it does not exist (this may be due to an aborted migration or similar): table_name: #{table_name}, index_name: #{index_name}" # rubocop:disable Gitlab/RailsLogger
@@ -150,13 +146,6 @@ module Gitlab
         end
       end
 
-      # Only available on Postgresql >= 9.2
-      def supports_drop_index_concurrently?
-        version = select_one("SELECT current_setting('server_version_num') AS v")['v'].to_i
-
-        version >= 90200
-      end
-
       # Adds a foreign key with only minimal locking on the tables involved.
       #
       # This method only requires minimal locking

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -142,7 +142,6 @@ describe Gitlab::Database::MigrationHelpers do
         allow(model).to receive(:transaction_open?).and_return(false)
         allow(model).to receive(:index_exists?).and_return(true)
         allow(model).to receive(:disable_statement_timeout).and_call_original
-        allow(model).to receive(:supports_drop_index_concurrently?).and_return(true)
       end
 
       describe 'by column name' do