Revert "Merge pull request #15867 from devnexen/haiku_malloc_usable_size"

This reverts commit 3ef91233ca, reversing
changes made to 2839e35d1c.

lib/std/c/dragonfly.zig

@@ -21,7 +21,6 @@ pub extern "c" fn dl_iterate_phdr(callback: dl_iterate_phdr_callback, data: ?*an
 pub extern "c" fn lwp_gettid() c_int;
 
 pub extern "c" fn posix_memalign(memptr: *?*anyopaque, alignment: usize, size: usize) c_int;
-pub extern "c" fn malloc_usable_size(?*const anyopaque) usize;
 
 pub const pthread_mutex_t = extern struct {
     inner: ?*anyopaque = null,

lib/std/c/haiku.zig

@@ -1071,5 +1071,3 @@ pub const sigevent = extern struct {
 
 /// TODO refines if necessary
 pub const PTHREAD_STACK_MIN = 2 * 4096;
-
-pub extern "c" fn malloc_usable_size(?*anyopaque) usize;