Just use rm for helper command to remove storage

This allows to use any kind of `rm` in `$PATH` for the system tests.

Signed-off-by: Sascha Grunert <sgrunert@suse.com>

test/system/helpers.bash

@@ -67,7 +67,7 @@ function basic_teardown() {
     run_podman '?' pod rm --all --force
     run_podman '?'     rm --all --force
 
-    /bin/rm -rf $PODMAN_TMPDIR
+    command rm -rf $PODMAN_TMPDIR
 }