Remove reminders to use C89 for some source files

This is part of removing support for VS versions older than VS 2013.
See #2465 for details and feedback.

Related to #2465

src/CMakeLists.txt

@@ -237,17 +237,6 @@ if (UNIX AND NOT APPLE)
     endif()
 endif()
 
-# Make GCC warn about declarations that VS 2010 and 2012 won't accept for all
-# source files that VS will build (Clang ignores this because we set -std=c99)
-if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
-    set_source_files_properties(context.c init.c input.c monitor.c platform.c vulkan.c
-                                window.c null_init.c null_joystick.c null_monitor.c
-                                null_window.c win32_init.c win32_joystick.c win32_module.c
-                                win32_monitor.c win32_time.c win32_thread.c win32_window.c
-                                wgl_context.c egl_context.c osmesa_context.c PROPERTIES
-                                COMPILE_FLAGS -Wdeclaration-after-statement)
-endif()
-
 if (WIN32)
     if (GLFW_USE_HYBRID_HPG)
         target_compile_definitions(glfw PRIVATE _GLFW_USE_HYBRID_HPG)