Remove Rails monkey-patches now that we're using Rails 4.2.6

Signed-off-by: Rémy Coutable <remy@rymai.me>

config/initializers/monkey_patch.rb

-## This patch is from rails 4.2-stable. Remove it when 4.2.6 is released
-## https://github.com/rails/rails/issues/21108
-
-module ActiveRecord
-  module ConnectionAdapters
-    class AbstractMysqlAdapter < AbstractAdapter
-      # SHOW VARIABLES LIKE 'name'
-      def show_variable(name)
-        variables = select_all("select @@#{name} as 'Value'", 'SCHEMA')
-        variables.first['Value'] unless variables.empty?
-      rescue ActiveRecord::StatementInvalid
-        nil
-      end
-
-      
-      # MySQL is too stupid to create a temporary table for use subquery, so we have
-      # to give it some prompting in the form of a subsubquery. Ugh!
-      def subquery_for(key, select)
-        subsubselect = select.clone
-        subsubselect.projections = [key]
-
-        subselect = Arel::SelectManager.new(select.engine)
-        subselect.project Arel.sql(key.name)
-        # Materialized subquery by adding distinct
-        # to work with MySQL 5.7.6 which sets optimizer_switch='derived_merge=on'
-        subselect.from subsubselect.distinct.as('__active_record_temp')
-      end
-    end
-  end
-end
-
-module ActiveRecord
-  module ConnectionAdapters
-    class MysqlAdapter < AbstractMysqlAdapter
-      ADAPTER_NAME = 'MySQL'.freeze
-
-      # Get the client encoding for this database
-      def client_encoding
-        return @client_encoding if @client_encoding
-
-        result = exec_query(
-          "select @@character_set_client",
-          'SCHEMA')
-        @client_encoding = ENCODINGS[result.rows.last.last]
-      end
-    end
-  end
-end