modules.genesis._bootstrap_yum: rm unused string format args

```
************* Module salt.modules.genesis
salt/modules/genesis.py:131: [W1304(unused-format-string-argument), _bootstrap_yum] Unused format argument 'confs'
salt/modules/genesis.py:132: [W1304(unused-format-string-argument), _bootstrap_yum] Unused format argument 'confs'
```