Removed GLFW_SYSTEM_KEYS from the GLFW API

Rationale: Disabling system commands is inherently
dangerous, and should not be encouraged. Also, it's very
difficult to define and implement a reliable and
consistent cross-platform mechanism.

src/window.c

@@ -297,7 +297,6 @@ GLFWAPI GLFWwindow glfwCreateWindow(int width, int height,
     window->mode       = mode;
     window->resizable  = wndconfig.resizable;
     window->cursorMode = GLFW_CURSOR_NORMAL;
-    window->systemKeys = GL_TRUE;
 
     // Open the actual window and create its context
     if (!_glfwPlatformCreateWindow(window, &wndconfig, &fbconfig))