Remove too verbose debug logging

By fluent#3185, log throttling per file feature was merged, but
it also introduce too much debug logging code in #handle_notify.

Before:

  def handle_notify
     ...
     while true
       ...
       @log.debug("reading file: ...")
       ...
     end
     ...

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

lib/fluent/plugin/in_tail.rb

@@ -993,7 +993,6 @@ def handle_notify
                     @fifo << data
                     @fifo.read_lines(@lines)
 
-                    @log.debug("reading file: #{@path}")
                     if limit_bytes_per_second_reached? || should_shutdown_now?
                       # Just get out from tailing loop.
                       read_more = false