Removing light switch from 3-way setup
Turn off all power before starting any work. OK switch B can remain in place but you have to determine which of the two orange travelers is hot when switch B is in the top (ON) position, then disconnect and tape the other orange wire. Remove switch A and wire nut the yellow wire to the hot orange that you did not disconnect from the other switch. Tape the other orange. You'll need to get the correct cover plate for switch A to cover up the unused gang.
It's always a good idea to put a note in each of the two boxes you worked in describing what you did for future reference. It might just help you, or a new owner, in the future.