Chris tells us ... he decided to dismiss the case because he understands the importance of family and grandparents. For her part Shar's happy, of course, especially because she feels he was dragging her good name through the mud.

We broke the story ... the "Zoey 101" star accused Shar -- his baby mama's mama -- of hitting his 2-year-old daughter, Mariah, and leaving her with a 4-inch "razor cut" on her forehead. A judge ordered her to stay 50 yards away from the child temporarily, but that's no more. According to the court clerk ... a judge granted Chris' request for dismissal.

What's unclear ... if Massey really believed Shar was abusive to his kid -- something she adamantly denies -- why'd he drop it?.