This effect is not magical at this point, just, as the dictionary definition puts it,
a lack of energy and enthusiasm.
So, since this is merely an exhaustion equivalent, dispel magic and antimagic field can not get rid of this effect, and if used during the spell's duration would simply end the spell and cause the exhaustion to happen sooner.
However, there is still a way to get rid the exhaustion, although you would have to talk to your DM about it. The text on the Greater Restoration spell says:
You can reduce the target's exhaustion level by one...
Since this is a form of exhaustion the argument could be made that this lethargy counts equivalent to a level of exhaustion, but due to the wording of the Haste spell, this solution can't be considered RAW, so if your DM doesn't allow it, then there is no other way to get rid of these effects using spells, although you can talk to your DM about finding a magic item to help you as well.