Remove Gnulib explicit_bzero module

Remove Gnulib explicit_bzero module

It’s no longer needed, as it is now merely a thin layer around
C23-style memset_explicit and we might as well just call the
C23 standard function; that’s more forward-looking.
* admin/merge-gnulib (GNULIB_MODULES):
Remove explicit_bzero, and add memset_explicit.
* lib/explicit_bzero.c, m4/explicit_bzero.m4:
Remove these files, which were copied from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/gnutls.c (gnutls_symmetric_aead):
Use memset_explicit instead of explicit_bzero.