region: Don't leak GFile objects

Creating the restart file leaked GFile objects. It also stored the path in the
object, which is unnecessary as it can just be generated on demand.

This leak has was introduced when the feature was in
db551f1c
11 jobs for region-restart-file-leak in 3 minutes and 58 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #100959
build

00:02:20

 
  Test
passed #100961
asan

00:01:37

passed #100960
test

00:00:41

 
  Manual
manual #100966
allowed to fail manual
armv8
manual #100962
allowed to fail manual
flatpak
manual #100965
allowed to fail manual
msan
manual #100967
allowed to fail manual
ppc64le
manual #100963
allowed to fail manual
tsan
manual #100964
allowed to fail manual
ubsan
 
  Review
manual #100968
allowed to fail manual
review
manual #100969
allowed to fail manual
stop_review