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How loud is this waterfall, and what extent of deafness are we talking about? Assuming some similarity to human hearing a prolonged or repeated exposure to volume over 85dB can cause hearing loss: the louder the sound, the shorter the time for noise-induced hearing loss to occur.

Evolution doesn't automatically get rid of useless features. (See the human appendix for an example.) You need evolutionary pressure - some factor that disadvantages individuals who have the feature from successfully mating and passing on their DNA. So unless the ability to hear (or possibly the consequences of constantly being deafened) became an evolutionary disadvantage, there is no real reason that the ability would be lost. Humans who become deaf are not really any less likely to be able to pass on their genes. If a feature is to become universal in a reasonably short evolutionary timescale it requires the selective pressure to be very strong, so that almost anyone without the feature (in this case genetic deafness) would be unable to survive to mating age.

Possible factors that might make the ability to hear become an evolutionary disadvantage might be if the pain of constantly being deafened or the effect of tinnitus is sufficiently distracting to interfere with effective hunting, or if constant burst eardrums act as a vector for infection (this would require a much, much louder waterfall - around 165dB in humans - and it's not clear that could come from a physically realistic waterfall). However the evolutionary result might as easily be thicker eardrums that result in a less sensitive but more robust sense of hearing that was no longer deafened by the waterfall. Hearing works over a very great range of volume anyway (the response is logarithmic) so some loss of sensitivity at the lower end to provide robustness against the loud waterfall might still allow them to hear everyday volumes.

The scenario raises other questions: for one, why does this race have to live (or why has it chosen to live) behind this particular waterfall, which on the face of it seems a rather unpleasant place.