Wayland: Remove gamma-related TODOs

Commit 9c513346ad ("Gamma will never be
supported on Wayland") made it clear that it cannot be implemented, so
this removes the TODO markers and rewords the error messages.

Related to #1387.

src/wl_monitor.c

@@ -182,17 +182,15 @@ void _glfwPlatformGetVideoMode(_GLFWmonitor* monitor, GLFWvidmode* mode)
 
 void _glfwPlatformGetGammaRamp(_GLFWmonitor* monitor, GLFWgammaramp* ramp)
 {
-    // TODO
     _glfwInputError(GLFW_PLATFORM_ERROR,
-                    "Wayland: Gamma ramp getting not supported yet");
+                    "Wayland: Gamma ramp access it not available");
 }
 
 void _glfwPlatformSetGammaRamp(_GLFWmonitor* monitor,
                                const GLFWgammaramp* ramp)
 {
-    // TODO
     _glfwInputError(GLFW_PLATFORM_ERROR,
-                    "Wayland: Gamma ramp setting not supported yet");
+                    "Wayland: Gamma ramp access is not available");
 }