Win32: Revert an unneeded fix

This fix is not needed.

https://docs.microsoft.com/en-us/windows/win32/inputdev/wm-unichar
> An application should return zero if it processes this message.

src/win32_window.c

@@ -893,7 +893,7 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
             }
 
             _glfwInputChar(window, (uint32_t) wParam, getKeyMods(), GLFW_TRUE);
-            return TRUE;
+            return 0;
         }
 
         case WM_KEYDOWN: