neorados_pool_test: Erase a pool name from local map during delete_pool

neorados_pool_test: Erase a pool name from local map during delete_pool

The test case itself call CoTearDown function by the end of every test case
and the function call delete_pool on the basis of pool name exists in
local set <created_pools> and if the pool does not exist during CoTearDown
it throws an exception so the test case expects the pool should be exists
before leaving the test case.

Solution: During delete_pool erase a pool name from a local map
          and CoTearDown iterate over local copy instead of the
          orignal copy.

Fixes: https://tracker.ceph.com/issues/69405
Signed-off-by: Mohit Agrawal <moagrawa@redhat.com>