iwlwifi: mvm: don't fail when removing a key from an inexisting sta

The iwl_mvm_remove_sta_key() function handles removing a key when the
sta doesn't exist anymore.  Mistakenly, this was changed to return an
error while fixing another bug.

If the mvm_sta doesn't exist, we continue normally, but just don't try
to remove the igtk key.

Fixes: cd4d23c1 ("iwlwifi: mvm: Fix removal of IGTK")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>