[web-animations-2] clearing the "auto-align start time" flag

[graouts]

The specification for the procedure to auto-align the start time does not indicate that the auto-align start time flag should be cleared. I don't see anywhere else in the Web Animations Level 2 spec that indicates that it should be cleared either. It seems to be this should be the last step of this procedure.

[graouts]

Cc @kevers-google, @andruud and @flackr.

[graouts]

I now think that the spec is correct. The "auto-align the start time" is meant to be sticky such that other changes that would affect the start time, say a change in attachment range or the metrics of the scroll container or its content, will auto-align it past animation readiness.