Is there any way to filter or ignore messages written to log?
In postgresql.conf the parameter log_checkpoints (boolean) causes checkpoints and restartpoints to be logged in the server log. Some statistics are included in the log messages, including the number of buffers written and the time spent writing them. This parameter can only be set in the postgresql.conf file or on the server command line. The default is off.

Not sure if this is the same thing, is yours commented or not?

#log_checkpoints = off


And anyway if you viewed your log using something like Glogg you could set an exclude pattern like: 

^(?!.*incomplete startup packet).*$ 


As simple as that.