swift: use statsd_default_sample_rate default

with the upgrade to icehouse completed this setting is no longer recommended
(see also this commit: https://review.openstack.org/gitweb?p=openstack%2Fswift.git;a=commitdiff;h=8801b7409030a1fb36fefcabd38af190bcb38c95)
and thus drop it otherwise it will skew the graphite metrics downwards

Change-Id: I856cee5aca08f092e49905ddce03340a0c347447
Merge "swift: use statsd_default_sample_rate default" into production