glib-aux: drop usage of malloc_usable_size() in nm_free_secret()

The idea of nm_free_secret() is to clear the secrets from memory. That
surely is some layer of extra snake oil, because we tend to pass secrets
via D-Bus, where the memory gets passed down to (D-Bus) libraries which
have no idea to keep it private. Still...

But turns out, malloc_usable_size() might not actually be usable for
this. Read the discussion at [1].

Stop using malloc_usable_size(), which seems unfortunate.

There is probably no secret relevant data after the NUL byte anyway,
because we tend to create such strings once, and don't rewrite/truncate
them afterwards (which would leave secrets behind as garbage).

Note that systemd's erase_and_free() still uses malloc_usable_size()
([2]) but the macro foo to get that right is terrifying ([3]).

[1] systemd/systemd#22801 (comment)
[2] https://github.com/systemd/systemd/blob/11c0f0659ecd82572c2dc83f3b34493a36dcd954/src/basic/memory-util.h#L101
[3] systemd/systemd@7929e18

Fixes: d63cd26 ('shared: improve nm_free_secret() to clear entire memory buffer')

src/libnm-glib-aux/nm-secret-utils.c

@@ -39,24 +39,10 @@ nm_explicit_bzero(void *s, gsize n)
 void
 nm_free_secret(char *secret)
 {
-    gsize len;
-
     if (!secret)
         return;
 
-#if GLIB_CHECK_VERSION(2, 44, 0)
-    /* Here we mix malloc() and g_malloc() API. Usually we avoid this,
-     * however since glib 2.44.0 we are in fact guaranteed that g_malloc()/g_free()
-     * just wraps malloc()/free(), so this is actually fine.
-     *
-     * See https://gitlab.gnome.org/GNOME/glib/commit/3be6ed60aa58095691bd697344765e715a327fc1
-     */
-    len = malloc_usable_size(secret);
-#else
-    len = strlen(secret);
-#endif
-
-    nm_explicit_bzero(secret, len);
+    nm_explicit_bzero(secret, strlen(secret));
     g_free(secret);
 }