Short answer
There is no research into whether it is possible to unlearn how to sleep in the dark. But it is likely best not to unlearn how to sleep in the dark. What is known from prior research is that you sleep better in a dark room than in a bright room, which inherently makes it healthier!
Longer answer
Why do you sleep better in the dark?
The suprachiasmatic nucleus (SCN) in the brain receives information from the eyes about how light or dark your environment is, and translates this into ‘time signals’. The SCN is therefore also called the biological clock. One of the things the SCN does when it is dark, is to stimulate the production of melatonin via the pituitary gland. This peak in the hormone melatonin, during the dark, is perceived as a signal by the body to inform it it is nighttime. For animals that active during daytime, such as humans, the rise in melatonin is a signal for your body and your brain to go into rest mode and to go to sleep. Light from the environment where you find your rest can disrupt this process and is one of the most important causes of sleep issues. Having light in your bedroom, such as your phone or even the light produced from a digital alarm clock, can suppress the production of melatonin. This prevents the body and the brain from getting optimal rest, and you wake easier. One person is more sensitive to ambient light than another but, in general, children are more sensitive to light than adults. For children, it is still important to ensure a dark setting for them to sleep in.

The suprachiasmatic nucleus (SCN) receives information about light in your environment and ensures that the rest of your brain and body respond accordingly. Source: File:Circadian rhythm labeled.jpg – Wikimedia Commons
How do you get good sleep?
As explained above, you sleep better when you have as little light as possible in your sleeping surroundings, and see as little light as possible when you go to sleep. Apart from ensuring your room is as dark as possible, it is also recommended not to be on your phone right before going to sleep. If you were to do so, you need to make sure the brightness of your screen is as low as possible, or even engage a blue light filter. Your SCN is especially sensitive to the blue light produced by your phone. Other good sleeping conditions that can help improve the quality of your sleep are to go to bed at the right time (when you are tired), sleep sufficiently (sleep until you wake naturally), and to sleep in a cooler room, instead of a warmer one. Such habits will habituate your biological clock and your body to differentiating between a wakeful state and a sleeping state. That is how you develop a clear day and night rhythm as a child, and this increases the likelihood for good sleep as an adult.
Read more?
Synchronizing effects of melatonin on diurnal and circadian rhythms – ScienceDirect Scientific article about the effect of melatonin on your day/night rhythm.
Let there be no light: the effect of bedside light on sleep quality and background electroencephalographic rhythms – ScienceDirect Scientific article about how the light from a digital alarm clock can influence you.