remove redundant copy of test_cache_base_dir_exists

Second copy of RbdTestCase.test_cache_base_dir_exists was accidentally
introduced in https://review.openstack.org/82840.

Change-Id: I16b2e472b2499f01379fdca9c860da0b95e291d8
Closes-bug: #1302211
Signed-off-by: Dmitry Borodaenko <dborodaenko@mirantis.com>
Merge "remove redundant copy of test_cache_base_dir_exists"