Check `ignore` value for legacy configuration

[quinnmil]

Changes

• Fixes a bug where if the only sampling configuration set was the ignore value, we weren't checking for this in the any_sampling check.
• Adds a test for this case.

[quinnmil]

Realized that the changes here are redundant. This has already been supported via this:

        self.ignore_endpoints = set(
            config.value("ignore_endpoints") + config.value("ignore")
        )

So we're removing extra logic, but leaving in the new test.