how to remove other linux os and stay with ubuntu only

Try this:

In a Ubuntu session, open a terminal,

Press Ctrl+Alt+T

Run it:

sudo -i
apt-get update
apt-get install --reinstall grub-pc grub-pc-bin grub2-common grub-common
grub-mkconfig -o /boot/grub/grub.cfg
grub-install --root-directory=/mnt /dev/sda
grub-install --recheck /dev/sda
fdisk -l

By fdisk identify the Fedora partitions, are suppose /dev/sda7 and /dev/sda8, continue running:

umount /dev/sda7
umount /dev/sda8
gparted

In gparted, select the hard disk, delete Fedora partitions y and to create a new, file system ext4, You can mount wherever you want in /media, /mnt, /home/user/.

Close gparted and run:

sudo update-grub