pathbar: Use flat style class

We use custom CSS that undecorates pathbar buttons in Adwaita. However,
this doesn't apply to HighContrast, which keeps button borders, and it
looks bad because there are slashes between them.

While some custom styling is still needed, some can be replaced using
the public "flat" class, with the benefit of removing the button
borders in HighContrast (as well as any 3rd party custom theme that
supports the "flat" class).

Do that. Also, remove the now pointless "linked" class.

Fixes: #661
3 jobs for 661-broken-pathbar-w-high-contrast-on-nautilus-3-30 in 6 minutes and 53 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed flatpak #112485

00:06:00

 
  Deploy
passed review #112486

00:00:53

failed stop_review #112487
allowed to fail manual

 
Name Stage Failure
failed
stop_review Deploy There has been a missing dependency failure
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