Revert "Add async dispatch event flag to Chromedriver perform action"

[jgraham]

This breaks the wdspec tests by causing them to send a nonstandard payload for actions.

[jgraham]

To clarify: changing tests that are intended to check that a specific message sent over the wire results in specific behaviour in the browser to instead send meaningfully different messages over the wire is inherently a breaking change. The fact that the extra parameters should be ignored by conforming implementations is immaterial. Indeed the only possible valid test in this case would be one that checked this parameter didn't have any effect.