build-aux: remove some syntax-check cruft

build-aux: remove some syntax-check cruft

sc_proper_name_utf8_requires_ICONV looks for Makefile.am files, so is
not going to work correctly with meson, nor did we ever use the GNULIB
"proper_name_utf8" function.

The 'today' variable is not referenced anywhere.

The 'writable-files' target is not used anywhere

sc_prohibit_reversed_compare_failure only checks 'init.sh' which does
not exist in libvirt.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>