Remove gtk-sharp2 from prereq list

gtk-sharp2 is used to build GUI version of Renode. We only use console
version in our project.

Change-Id: I1727c1ee419c3df4c852f9d88da6935585b990e4

install-prereqs.sh

diff --git a/install-prereqs.sh b/install-prereqs.sh
index d86c893..6b694dd 100755
--- a/install-prereqs.sh
+++ b/install-prereqs.sh
@@ -36,7 +36,6 @@
     fzf
     gdb
     git
-    gtk-sharp2
     graphviz
     haskell-stack
     lcov