How Can I Disable the Global Shortcut CTRL + SHIFT + U in Linux Mint?

Backstory: some Linux distributions setup a keyboard shortcut to let you enter Unicode characters, and they use CTRL + SHIFT + U as that shortcut. As a programmer, I want to instead use that shortcut in my code editor, so I'd like to disable the system one ... but I can't seem to figure out what program is responsible for it. Everything I've found (eg. in this question about *re-enabling* that shortcut: https://unix.stackexchange.com/questions/258668/how-to-re-enable-the-ctrlshiftu-shortcut-in-cinnamon-mint-17-3) suggests that it's dependent on my IME (input method editor). But when I went to "Input Method" (in the system menu), it said I didn't have an IME selected. I then tried switching to the Fcitx IME (since I *would* like to be able to type Japanese characters), but that didn't help. Also, I went through all of the Fcitx configuration, and couldn't find any mention of a CTRL + SHIFT + U shortcut in it. My question is: how can I disable this global CTRL + SHIFT + U shortcut? Preferably with the Fcitx IME, but if I have to disable it I can.