windows: Drop explicit `read_in_progress` flag
windows: Drop explicit `read_in_progress` flag

Since the `ov` field is now only set when we have a read in progress,
and thus effectively serves as an indicator of that state, the explicit
flag became redundant: we can just check the presence of `ov` instead.

(And since a check is performed implicitly when unwrapping the `ov`
value, some of the explicit asserts on `read_in_progress` can be dropped
entirely.)

servo/ipc-channel