network: Fix mtu label not requesting enough space

Remove the size group where mtu label belongs, so that it can request
enough space when it becomes visible. Since the size group is removed,
filler widget is no longer useful. Remove it as well.
11 jobs for fix-mtu-label in 6 minutes and 8 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #354766
build

00:04:39

 
  Test
passed #354768
asan

00:01:28

passed #354767
test

00:00:57

 
  Manual
manual #354773
allowed to fail manual
armv8
manual #354769
allowed to fail manual
flatpak
manual #354772
allowed to fail manual
msan
manual #354774
allowed to fail manual
ppc64le
manual #354770
allowed to fail manual
tsan
manual #354771
allowed to fail manual
ubsan
 
  Review
manual #354775
allowed to fail manual
review
manual #354776
allowed to fail manual
stop_review