region: Remove duplicate cancellable code

There was a GCancellable used for IBus operations that was generated each time
an operation was done. The cancellable was only used to cancel such operations
when the panel is closed. We can just use the shared cancellable for this.
11 jobs for region-duplicate-cancellable in 16 minutes and 2 seconds (queued for 4 minutes and 32 seconds)
Status Job ID Name Coverage
  Build
passed #100553
build

00:02:21

 
  Test
passed #100556
armv8

00:12:57

passed #100555
asan

00:01:39

passed #100557
allowed to fail
ppc64le

00:11:48

passed #100554
test

00:00:40

 
  Manual
manual #100558
allowed to fail manual
flatpak
manual #100561
allowed to fail manual
msan
manual #100559
allowed to fail manual
tsan
manual #100560
allowed to fail manual
ubsan
 
  Review
manual #100562
allowed to fail manual
review
manual #100563
allowed to fail manual
stop_review