Remove quickplot unit label for time reference coordinates. Closes #615.

This PR allows 1D plots in quickplot to omit the units part of the axis label when the coordinate it represents is given as a time reference, and hence will have been converted to an absolute date/time.

This change applies only to 1D plots because handling of time coordinates in 2D plots is not yet implemented, so the unit is still relevant there. This is the reason the implementation modifies `_get_titles` rather than `_title`.