view-switcher: Drop the icon-size property

We don't want to recommend changing it, so let's just not allow do to
so.

Fixes GNOME/libhandy#159
11 jobs for view-switcher-drop-icon-size in 13 minutes and 29 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #842819

00:00:30

passed build-debian-clang #842821

00:04:31

passed build-debian-gcc #842820

00:01:58

passed build-fedora-gcc #842822

00:03:17

passed build-flatpak #842823
flatpak

00:01:46

passed build-flatpak-glade #842824
flatpak

00:03:16

 
  Test+Docs
passed build-gtkdoc #842825

00:02:09

passed unit-test #842826

00:03:41

44.1%
 
  Package
passed package-debian-buster #842827

00:02:55

 
  Test Package
passed autopkgtest-debian-buster-package #842828

00:02:19

passed lintian-debian-buster-package #842829

00:01:22