How do you fix the skimmer flaps on an inground concrete pool?

Actually, they aren't a bad thing to remove all together. Skimmer flaps are a notorious place for algae, especially black or mustard, and bacteria to hide and grow, which makes reoccurance terribly probable. Removing them all together will not harm the performance of your pool.

I beg to differ with the answer above -

The skimmer works because the water jets create a flow which pushes the water past the skimmer. The skimmer is pulling water down which causes debris on the top of the water to get caught in the basket. The skimmer flap(s) limits the amount of water being sucked in by the skimmer causing more suction and making the skimmer more efficient. It is important for the skimmer doors, or "weirs" as they are called, to be in place for your skimmer to work properly.

Try it for yourself. With the skimmer running, place your hand, a brick or some other obstruction where the skimmer door should be. Restrict the water flow a bit in some way. You will see a much larger volume of water being sucked into the skimmer than with no door at all. If you already have a flap or wier in place, do the reverse. With the skimmer running, hold the door completely open and see hose the flow virtually ceases.

Replacement weirs or flaps can easily be found at your pool store. Just take your old one with you or measure the opening where it should sit. Little pegs on the sides of the replacements are spring loaded to keep it properly situated so it is very important that the new weir be exactly where you want it before pulling the pins securing the pegs.

I hope this has helped!