remove window.web3

As I understand it, `window.web3` has been removed from metamask https://github.com/MetaMask/metamask-extension/pull/9156

It looks like we are still using window.web3 in some places https://github.com/gitcoinco/web/search?q=window.web3&type=code

which is causing this message to pop in some cases (depending maybe on Metamask version?)

**To Reproduce**
1) log out of metamask completely and make sure you're running the latest version
2) log out of github and gitcoin
3) clear browser cache
4) login to Gitcoin.co and trigger a web3 interaction (for example, go here https://stage.gitcoin.co/settings/tokens and enable some token).

**Expected behavior**
No metamask warning. I'm not 100% sure of the reason for the warning message but it seems likely it's related to use of `window.web3`.

**Desktop (please complete the following information):**

- OS: OSX
- Browser:FF, Chrome