Remove Rugpull Migrator Code

Problem statement: Inherited from the sushiswap era PancakeSwap has a migrator code present in the contract. This means the chef can migrate away all staked assets. He can do this within 6 hours after the timelock.

This is the same issue as the recent Popcorn Swap RugPull.

Solution: When the website is down users could use emergency withdrawal to withdrawal all their assets.

To remove this migrator code PancakeSwap will provide extra security to its users.

Source 1 "Link to migrator code": https://github.com/pancakeswap/pancake-farm/blob/master/contracts/MasterChef.sol

Source 2 "Popcorn swap rug pull": https://www.bsc.news/post/popcornswap-liquidity-siphon-rugpull