Vou optar por ficar somente com o Ubuntu, e por isso, tem como eliminar o Kubuntu sem reinstalar tudo?
Sim.
Mas como disse o @CelticWarrior, vai depender de como vc configurou os dois.
Coloca a saída dos comandos abaixo que poderemos te orientar melhor:
lsblk
sudo os-prober
sudo parted -l
Vamos lá:
__________________________
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop0 7:0 0 9,1M 1 loop /snap/canonical-livepatch/95
loop1 7:1 0 240,8M 1 loop /snap/chromium/1373
loop2 7:2 0 240,8M 1 loop /snap/chromium/1382
loop3 7:3 0 142,3M 1 loop /snap/code/49
loop4 7:4 0 161,4M 1 loop /snap/gnome-3-28-1804/128
loop5 7:5 0 33M 1 loop /snap/chromium-ffmpeg/17
loop6 7:6 0 142,8M 1 loop /snap/code/48
loop7 7:7 0 55,3M 1 loop /snap/core18/1885
loop8 7:8 0 97,7M 1 loop /snap/core/10126
loop9 7:9 0 97,8M 1 loop /snap/core/10185
loop10 7:10 0 255,6M 1 loop /snap/gnome-3-34-1804/36
loop11 7:11 0 116,1M 1 loop /snap/icloud-for-linux/175
loop12 7:12 0 140K 1 loop /snap/gtk2-common-themes/13
loop13 7:13 0 162,9M 1 loop /snap/gnome-3-28-1804/145
loop14 7:14 0 217,9M 1 loop /snap/gnome-3-34-1804/60
loop15 7:15 0 62,1M 1 loop /snap/gtk-common-themes/1506
loop16 7:16 0 73,1M 1 loop /snap/simplenote/384
loop17 7:17 0 31M 1 loop /snap/snapd/9607
loop18 7:18 0 31M 1 loop /snap/snapd/9721
loop19 7:19 0 84,9M 1 loop /snap/simplenote/407
loop20 7:20 0 145,5M 1 loop /snap/opera/98
loop21 7:21 0 260,7M 1 loop /snap/kde-frameworks-5-core18/32
loop22 7:22 0 62,3M 1 loop /snap/rambox/14
loop23 7:23 0 145,5M 1 loop /snap/opera/99
loop24 7:24 0 20M 1 loop /snap/photoscape/33
loop25 7:25 0 20M 1 loop /snap/photoscape/43
loop26 7:26 0 256,6M 1 loop /snap/wps-office-all-lang-no-internet/1
loop27 7:27 0 193,4M 1 loop /snap/mailspring/488
loop28 7:28 0 55,4M 1 loop /snap/core18/1932
loop29 7:29 0 62,4M 1 loop /snap/rambox/16
loop30 7:30 0 55,3M 1 loop /snap/teams-for-linux/105
loop31 7:31 0 86,8M 1 loop /snap/shotcut/71
loop32 7:32 0 82M 1 loop /snap/shotcut/94
loop33 7:33 0 290,4M 1 loop /snap/vlc/1700
loop34 7:34 0 231,4M 1 loop /snap/wine-platform-runtime/179
loop35 7:35 0 132K 1 loop /snap/gtk2-common-themes/9
loop36 7:36 0 231,8M 1 loop /snap/wine-platform-runtime/183
loop37 7:37 0 355,4M 1 loop /snap/pycharm-community/214
loop38 7:38 0 49,8M 1 loop /snap/snap-store/467
loop39 7:39 0 67,6M 1 loop /snap/sublime-text/85
loop40 7:40 0 50,7M 1 loop /snap/snap-store/481
loop41 7:41 0 215M 1 loop /snap/wine-platform-5-stable/5
loop42 7:42 0 116,1M 1 loop /snap/icloud-for-linux/183
loop43 7:43 0 215M 1 loop /snap/wine-platform-5-stable/10
sda 8:0 0 111,8G 0 disk
├─sda1 8:1 0 512M 0 part /boot/efi
├─sda2 8:2 0 1K 0 part
├─sda3 8:3 0 513M 0 part
├─sda5 8:5 0 87,9G 0 part /
└─sda6 8:6 0 22,9G 0 part
sr0 11:0 1 1024M 0 rom
_______________________________
/dev/sda6:Ubuntu 20.04.1 LTS (20.04):Ubuntu:linux
_______________________________
Modelo: ATA KINGSTON SA400S3 (scsi)
Disco /dev/sda: 120GB
Tamanho do setor (lógico/físico): 512B/512B
Tabela de Partição: msdos
Opções de disco:
Número Início Fim Tamanho Tipo Sistema de arquivos Opções
1 1049kB 538MB 537MB primary fat32 boot
2 539MB 119GB 119GB extended
5 539MB 94,9GB 94,4GB logical ext4
6 94,9GB 119GB 24,6GB logical ext4
3 119GB 120GB 538MB primary fat32