Switch ubuntustudio to generic kernel

The lowlatency kernel will eventually undergo deprecation. Rather than
wait for such a time to happen and be reactive, Ubuntu Studio would
rather be proactive about this now that the generic kernel can act as a
lowlatency kernel with certain command line parameters as outlined by
https://discourse.ubuntu.com/t/fine-tuning-the-ubuntu-24-04-kernel-for-low-latency-throughput-and-power-efficiency/44834.

As such, we have modified our `ubuntustudio-lowlatency-settings`
package, which installs `/etc/default/grub.d/ubuntustudio.cfg` with the
following line:

-GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT threadirqs"
+GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT preempt=full
nohz_full=all threadirqs"

Additionally, that same file used to set "GRUB_FLAVOUR_ORDER" which is
no longer needed.

live-build/auto/config

@@ -951,7 +951,7 @@ case $PROJECT in
 		# By default Ubuntu Studio now ships the new installer.
 		UCFLAVOUR="Ubuntu Studio"
 		STANDARD_TASKS="ubuntustudio-desktop ubuntustudio-audio ubuntustudio-graphics ubuntustudio-video ubuntustudio-photography"
-		KERNEL_FLAVOURS=lowlatency
+		KERNEL_FLAVOURS=generic
 		do_layered_desktop_image
 		;;