build: Drop pointless -Wformat-security warning flag

We already set -Wformat=2, which implies -Wformat-security, so there’s
no need to test for and set -Wformat-security separately.

The test for -Wformat-security never worked anyway, since gcc complains
if it’s specified without also setting -Wformat to some value. The
complaint causes configure.ac/meson.build to assume the option doesn’t
work.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/656

configure.ac

@@ -3408,7 +3408,7 @@ AS_IF([test "x$enable_compile_warnings" != xno], [
    -Wno-bad-function-cast \
    -Werror=declaration-after-statement \
    -Werror=missing-prototypes -Werror=implicit-function-declaration \
-   -Werror=pointer-arith -Werror=init-self -Werror=format-security \
+   -Werror=pointer-arith -Werror=init-self \
    -Werror=format=2 -Werror=missing-include-dirs])
 ])
 AC_SUBST(GLIB_WARN_CFLAGS)