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@lynob

I guess your "dnf remove kernel-core-x.y.z" probability doesn't work as expected cause you did a "sudo dnf remove kernel-4.x" before.

in this special case you need to do "rpm -qa|grep kernel" and "dnf remove ..." all the parts left over from the above kernel aswell  [1]

afterwards you need to do the "sudo grub2-mkconfig...."

after that it seems very unlikely that you are able to boot that removed kernel furthermore !

[1]
on my box:
rpm -qa | grep kernel gives

kernel-4.13.9-300.fc27.x86_64
kernel-core-4.13.9-300.fc27.x86_64
kernel-headers-4.13.9-300.fc27.x86_64
kernel-modules-4.13.9-300.fc27.x86_64

P.S.
I do dnf remove kernel-core-x.y.z cause all other kernel packages (headers, modules and the kernel itself) depend on kernel-core. dnf realize that and removes the depending parts aswell. I'm somewhat lazy !

@sideburns regarding GUI (I believe [ I don't use it] there is one already called "gnome-software"

but what is so difficult in
1. to open a terminal and
2. rpm -qa|grep kernel-core and
3. sudo dnf remove kernel-core-x.y.z and 
4. sudo  sudo grub2-mkconfig -o /boot/grub2/grub.cfg && sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

Hints:
- double left click one kernel-core in the output of step 2
- key in "sudo dnf remove " and 
a) middle click to paste the before highlight kernel-core OR
b) to write that command further to read "sudo dnf remove kernel-c" and hit the tab key (command expansion) once or more times to select the right kernel-core