You can boot a live system with GParted included (such as a Trisquel live
system), run this administration program and delete the unnecessary
partitions.
Then, I am not 100% sure but I believe you can reboot and the GRUB entry for
Trisquel should still work. From a terminal in the Trisquel system, you can
then execute:
$ sudo update-grub
That should remove the Ubuntu entry from GRUB.
You can then use again GParted on the live system to enlarge partitions in
the space left by the deleted ones (maybe you can even do that in the first
place... but let us be safe).
If I am wrong (I do not think so but anyway), i.e., if you cannot reboot the
Trisquel system after removing the Ubuntu partitions, then you would have to
reinstall GNU GRUB from the live system.
