Merge single-use function _glfwFreeMonitors

All other uses were elminiated by _glfwInputMonitor.

src/internal.h

@@ -1065,10 +1065,6 @@ _GLFWmonitor* _glfwAllocMonitor(const char* name, int widthMM, int heightMM);
   */
 void _glfwFreeMonitor(_GLFWmonitor* monitor);
 
-/*! @ingroup utility
-  */
-void _glfwFreeMonitors(_GLFWmonitor** monitors, int count);
-
 /*! @ingroup utility
  */
 GLFWbool _glfwIsPrintable(int key);