docs/process: branching-checklist: Remove reference to qemu-ijw.git

docs/process: branching-checklist: Remove reference to qemu-ijw.git

Per [1], qemu-iwj.git was a clone of QEMU traditional on Ian's computer
for tagging QEMU trad.

The next section will provide tag for the official tree. So remove
the section about qemu-ijw.git.

[1] 25482.10006.140155.984629@chiark.greenend.org.uk

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: George Dunlap <george.dunlap@cloud.com>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>