subsys/settings: remove deprecated function

`settings_mount_fs_backend()` has been deprecated for
more than 2 releases, remove it.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
diff --git a/subsys/settings/include/settings/settings_file.h b/subsys/settings/include/settings/settings_file.h
index af361c2..d7695a5 100644
--- a/subsys/settings/include/settings/settings_file.h
+++ b/subsys/settings/include/settings/settings_file.h
@@ -32,11 +32,6 @@
 
 void settings_mount_file_backend(struct settings_file *cf);
 
-__deprecated static inline void settings_mount_fs_backend(struct settings_file *cf)
-{
-	settings_mount_file_backend(cf);
-}
-
 #ifdef __cplusplus
 }
 #endif