Fix ci-tasks container shell instructions

With the variable reorg in commit a452f1b, `CI_TASKS_TEST_ARGS`
already contains the volume mount and `--rm`, so it's not necessary any
more to repeat it -- in fact, that fails:

    Error: /anaconda: duplicate mount destination

Cherry-picked from master PR rhinstaller#2923

Related: rhbz#1885635

tests/README.rst

@@ -76,7 +76,7 @@ Logs from the run are stored in the ``tests`` folder.
 
 For debugging of the container please run the container as::
 
-    make container-ci CONTAINER_TEST_ARGS="--rm -ti -v .:/anaconda:Z --entrypoint /bin/bash"
+    make container-ci CONTAINER_TEST_ARGS="-it --entrypoint /bin/bash"
 
 This command will open bash inside the container for you with mounted
 current folder at the `/anaconda` path. This could be even convenient way