You can remove the bitterness from cucumbers by thinly slicing off both ends and then use one of the end pieces to rub it against the entire cucumber.
Doing this will almost always remove the bitterness from cucumbers even though it's perfectly safe to eat bitter cucumbers.
Cucumbers that have become bitter are still safe to eat although it might not be as tasty as non bitter cucumbers.
You can sometimes get rid of the bitterness in the cucumbers by thinly slicing off both ends of the cucumbers and rubbing once piece against the bitter cucumber.
I do this sometimes when I have bitter cucumbers and it does get rid of the bitterness in cucumbers.
You can safely eat the cucumbers though even when bitter but I recommend doing the above to get rid of the bitterness for a better tasting cucumber.