core-utils: delete dead fee docs

A very legacy version of fee calculation was still in
the readme. This commit deletes the information from
the readme.

packages/core-utils/README.md

@@ -24,32 +24,3 @@ $ yarn start
 $ yarn test
 $ yarn lint
 ```
-
-### L2 Fees
-
-`TxGasLimit` can be used to `encode` and `decode` the L2 Gas Limit
-locally.
-
-```typescript
-import { TxGasLimit } from '@eth-optimism/core-utils'
-import { JsonRpcProvider } from 'ethers'
-
-const L2Provider = new JsonRpcProvider('https://mainnet.optimism.io')
-const L1Provider = new JsonRpcProvider('http://127.0.0.1:8545')
-
-const l2GasLimit = await L2Provider.send('eth_estimateExecutionGas', [tx])
-const l1GasPrice = await L1Provider.getGasPrice()
-
-const encoded = TxGasLimit.encode({
-  data: '0x',
-  l1GasPrice,
-  l2GasLimit,
-  l2GasPrice: 10000000,
-})
-
-const decoded = TxGasLimit.decode(encoded)
-assert(decoded.eq(gasLimit))
-
-const estimate = await L2Provider.estimateGas()
-assert(estimate.eq(encoded))
-```