Revert "fluentd: Add un-escaping of control characters in JSON" (#7468)

Reverts grafana/loki#7396 according to discussing on PR

clients/cmd/fluentd/lib/fluent/plugin/out_loki.rb

@@ -278,7 +278,6 @@ module Fluent
           case @line_format
           when :json
             line = Yajl.dump(record)
-            line = line.gsub("\\n", "\n").gsub("\\r", "\r").gsub("\\s", "\s").gsub("\\t", "\t")
           when :key_value
             formatted_labels = []
             record.each do |k, v|