Remove jsoncpp from desktop Linux shell setup (flutter#8230)

The jsoncpp requirement was removed in the migration from FDE to this
repository, so it doesn't need to be installed by the script.

build/install-build-deps-linux-desktop.sh

@@ -8,5 +8,4 @@
 
 set -e
 
-sudo apt-get -y install libglfw3-dev libepoxy-dev libjsoncpp-dev libgtk-3-dev \
-    libx11-dev
+sudo apt-get -y install libglfw3-dev libepoxy-dev libgtk-3-dev libx11-dev