Reland "[dawn][headers] Remove obsolete ifdefs."

This is a reland of commit 1c4943b809ca59013ce0f84fc618e8940b419278

Original change's description:
> [dawn][headers] Remove obsolete ifdefs.
>
> - Since the breaking changes have been fixed in downstream users,
>   it should be safe to remove the ifdefs.
>
> Bug: 374263404, 369445924, 42241174
> Change-Id: Ia6894285b4f94a3aeefd2cb2479c219719449b25
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/228854
> Auto-Submit: Loko Kung <lokokung@google.com>
> Reviewed-by: Corentin Wallez <cwallez@chromium.org>
> Commit-Queue: Corentin Wallez <cwallez@chromium.org>

Bug: 374263404, 369445924, 42241174
Change-Id: Id806499aa3be5054b0b640786ced1eb19a4a56da
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/229094
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/generator/templates/api.h b/generator/templates/api.h
index 3a1cc8b..d7fada5 100644
--- a/generator/templates/api.h
+++ b/generator/templates/api.h
@@ -40,9 +40,6 @@
 #define WGPU_BREAKING_CHANGE_STRING_VIEW_LABELS
 #define WGPU_BREAKING_CHANGE_STRING_VIEW_OUTPUT_STRUCTS
 #define WGPU_BREAKING_CHANGE_STRING_VIEW_CALLBACKS
-#define WGPU_BREAKING_CHANGE_FUTURE_CALLBACK_TYPES
-#define WGPU_BREAKING_CHANGE_LOGGING_CALLBACK_TYPE
-#define WGPU_BREAKING_CHANGE_FLATTEN_LIMITS
 
 {% set API = metadata.c_prefix %}
 #if defined({{API}}_SHARED_LIBRARY)
@@ -79,8 +76,6 @@
 #define {{API}}_NULLABLE
 #endif
 
-#define WGPU_BREAKING_CHANGE_DROP_DESCRIPTOR
-
 #include <stdint.h>
 #include <stddef.h>