Accidentally removed grout behind bathtub caulk, necessary to replace?
Grout at that location does not last. I think tilers put it there to get the job "done" with only one session. I would think that a proper job would leave out grout there and just use long lasting caulk.

The best way to proceed would be to remove all the grout and seal with caulk or silicone sealer.

EDIT

But if you remove the grout be careful not to damage the finish on that tub.