highgui: Make GThread mandatory with GTK

The GThread API is available since more than 11 years with GLib, it is now safe
to assume that it is always available.

modules/highgui/misc/plugins/plugin_gtk/CMakeLists.txt

@@ -50,9 +50,3 @@ elseif(DEFINED GTK_VERSION)
 else()
   message(STATUS "GTK+: YES")
 endif()
-
-if(HAVE_GTHREAD)
-  message(STATUS "GThread : YES (ver ${GTHREAD_VERSION})")
-else()
-  message(STATUS "GThread : NO")
-endif()