replace get(key).is_none() with !contains_key()

replace get(key).is_none() with !contains_key()

Fixes the clippy warning:

warning: unnecessary use of `get(&user2).is_none()`
    --> pbs-config/src/acl.rs:1067:36
     |
1067 |                 assert!(node.users.get(&user2).is_none());
     |                         -----------^^^^^^^^^^^^^^^^^^^^^
     |                         |
     |                         help: replace it with: `!node.users.contains_key(&user2)`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_get_then_check

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>