chore: remove codecov token

It looks like 1a1a820 actually broke
codecov. Codecov is currently trying to diff all PRs against the commit
before that one, which leads to flagging previously-added lines of code
as needing coverage in the current PR.

Let's try removing the token and see if codecov is happy again.

Fixes #1124

.github/workflows/main.yml

@@ -51,5 +51,4 @@ jobs:
         if: matrix.node-version >= 10
         uses: codecov/codecov-action@v3
         with:
-          token: ${{ secrets.CODECOV_TOKEN }}
           fail_ci_if_error: true