haproxy: remove needless configuration for log.

because we anyway has never used log, but just use stderr ourput.
4 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:27:42

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:07:22

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

26:43:56

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

05:03:32