system cfg: remove mkpasswd MD5 format option

system cfg: remove mkpasswd MD5 format option

As SHA256 is now default, removing weak MD5 option.  C libraries now
all support the SHA methods.
    glibc 2.7+
    uclibc (bdd8362a88 package/uclibc: defconfig: enable sha-256...)
    musl 1.1.14+

One issue this would prevent, is a host tool issue with a FIPS enabled
system where weak ciphers/methods are disabled. It seems the crypt(3)
call is impacted by /proc/sys/crypto/fips_enabled (per crypt(3) man
page). It results in mkpasswd returning "(EPERM) crypt failed."
Rather then create a Buildroot host dependency check, this patch
removes the potential corner case from being selected.

Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>