Remove CircleCI folder from dtest

CircleCI configuration in this repo is never used and, thus, is not
up-to-date, which is confusing. This commit removes it. Configuration
from the main Cassandra repo is used for running dtest in CircleCI.

 patch by Ruslan Fomkin; reviewed by Brandon Williams, Michael Semb Wever for CASSANDRA-16853

.gitignore

@@ -10,3 +10,4 @@ last_test_dir
 upgrade
 html/
 doxygen/doxypy-0.4.2/
+.pytest_cache/