remove empty module tests.collector
remove empty module tests.collector

The test code for collector has been moved from tests/collector/test_service.py
to tests/test_collector.py, but the empty directory has been left.
This patch removes it.

ref-commitid: b6aab6658697016950309d1276fe0709be0c3fca
ref-changeid: I4c39ea74fc5cb59d487626b998eaed9c5cf30eff

Change-Id: Id5b3ee6b26ac8ae3627bbd3f8a1e73d90fd165ca

idegtiarov/ceilometer,maestro-hybrid-cloud/ceilometer,openstack/ceilometer,pczerkas/aodh,ityaptin/ceilometer,openstack/aodh,isyippee/ceilometer,chungg/aodh,eayunstack/ceilometer,sileht/aodh,sileht/aodh,pczerkas/aodh,fabian4/ceilometer,redhat-openstack/ceilometer,chungg/aodh,mathslinux/ceilometer,redhat-openstack/ceilometer,cernops/ceilometer,openstack/ceilometer,idegtiarov/ceilometer,ityaptin/ceilometer,openstack/aodh,isyippee/ceilometer,pkilambi/ceilometer,mathslinux/ceilometer,fabian4/ceilometer,Juniper/ceilometer,r-mibu/ceilometer,eayunstack/ceilometer,maestro-hybrid-cloud/ceilometer,cernops/ceilometer,Juniper/ceilometer,r-mibu/ceilometer,pkilambi/ceilometer