Consider removing DisparityImage header field

[tfoote]

This is a follow up to the Aggregate Foxy Message API Review

The DisparityImage has a header that's documented as it will likely be removed in a future release.

common_interfaces/stereo_msgs/msg/DisparityImage.msg

Lines 1 to 3 in 519e851

	# Separate header for compatibility with current TimeSynchronizer.
	# Likely to be removed in a later release, use image.header instead.
	std_msgs/Header header

Since that was several years ago are we ready to remove it now? The one question is whether the TimeSynchronizer can support introspecting into the internal image headers.

[tfoote]

Reviewing the useages of this message there's still a large fraction of the common use cases which leverage the message_fitler implementation which relies upon the root header. Removing that would break those use cases. And as such we should leave it be for now. The cost of the additional data is low compared to the size of the disparity data.