Information regarding Brassica juncea var. rugosa (芥菜) and it's bitter leaves

Bolted?

This is a mustard related to bok choi, napa cabbage etc. If mustard (or many other leafy plants) bolt and set flowers they can become bitter. One would think that they would not then be sold, but sellers can cut off the flowers and hope no-one notices. I once bought some like this at the farmers market and a different seller caught sight of it and pointed out where the flowers had been cut off.

I am not super averse to bitter so I ate it anyway.

why the leaves are bitter

They are bitter because animals dislike bitter taste. Many plants, including most plants which humans use as salad greens or leaf vegetables, are naturally bitter. They have evolved this way as a protection from being eaten.

Humans are trying to reduce the bitterness through selective breeding, and have been rather successful in recent decades, but this doesn't mean that every single plant you encounter will have lost its bitterness.

whether they are edible

Yes, plants sold as vegetables are edible, even exemplars which are bitter.

is it possible to remove the bitterness in the leaves

No, it isn't possible. For some bitter plants, you can do a salt water soak, but leafy greens are no longer tasty after a long soak. Once you bought a bitter plant, you have to live with its bitterness.

It's OK, you can eat it. Some Chinese people might actually prefer them going a bid bitter, similar to how many people love the bitter gourd.

Blanching it in the water and/or marinating it with salt reduces the bitterness, but it doesn't just go away. I suggest you try making dumplings with the vegetable after a blanche and light marination, if it does become a bit less bitter afterwards :-)