headers: Remove VK_LAYER_EXPORT

- It hasn't been handling windows (so it's been half implemented)
- It's a suboptimal method for exporting symbols for vulkan layers
since `--version-script` and `.def` files exists. Both of these
methods also don't risk name mangling.
- `--version-script` also has perf benefits since you can silence
exports from code you didn't intend to export

tests/vk_layer.c

@@ -1,7 +1,6 @@
 #include "vulkan/vk_layer.h"
 
-// Use helper macro intended for vulkan layers to export functions
-VK_LAYER_EXPORT int foobar()
+int foobar()
 {
     return 0;
 }