op-e2e: only add a signer key to l1

The current code adds the same signing key to all Ethereum clients
(including the L2 nodes).  I found this to be quite confusing when
parsing the code, and, it's unnecessary for the L2s to have signing
keys.

op-e2e/geth.go

@@ -300,6 +300,7 @@ func createGethNode(l2 bool, nodeCfg *node.Config, ethCfg *ethconfig.Config, pri
 		return nil, nil, err
 	}
 
+	if !l2 {
 		keydir := n.KeyStoreDir()
 		scryptN := 2
 		scryptP := 1
@@ -319,6 +320,7 @@ func createGethNode(l2 bool, nodeCfg *node.Config, ethCfg *ethconfig.Config, pri
 				return nil, nil, err
 			}
 		}
+	}
 
 	backend, err := eth.New(n, ethCfg)
 	if err != nil {
@@ -341,5 +343,4 @@ func createGethNode(l2 bool, nodeCfg *node.Config, ethCfg *ethconfig.Config, pri
 		}
 	}
 	return n, backend, nil
-
 }