Fix remove_local_wheels for no build wheels

[cdce8p]

Fix a small issue introduced with #794.

Without this PR, the code would raise a KeyError.

        ...
        for binary in exists:
            version = binary_package_map[binary]
            print(f"Found existing wheels for {binary}, removing local copy {version}")
>           for wheel in wheel_map[binary]:
E           KeyError: 'grpcio'

[frenck]

Thanks, @cdce8p 👍

../Frenck