Remove unused variable in fluentd output plugin (#4261)

I couldn't figure out why this variable is there. If I am missing any global variables by fluentd feel free to close this PR without any notice

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

@@ -343,10 +343,8 @@ def line_to_loki(record)
       # iterate through each chunk and create a loki stream entry
       def chunk_to_loki(chunk)
         streams = {}
-        last_time = nil
         chunk.each do |time, record|
           # each chunk has a unique set of labels
-          last_time = time if last_time.nil?
           result = line_to_loki(record)
           chunk_labels = result[:labels]
           # initialize a new stream with the chunk_labels if it does not exist