package/coreutils: Prevent overwriting of fakedate

When BR2_REPRODUCIBLE is set and host-coreutils needs to be built, the
fakedate script installed to 'host/bin/date' will be overwritten by
host-coreutils.

Besides, we do not need our host-coreutils for 'date' at all; we really
rely on the host system to provide it.

Unconditionally disable installing the 'date' binary in host-coreutils.

Note that we explicitly request only ln and realpath to be installed,
but the coreutils buildsystem does not strictly obey to that, as was
already noticed in 885e6fdb (package/coreutils: introduce a host
variant), which added that comment above HOST_COREUTILS_CONF_OPTS:

    # Explicitly install ln and realpath, which we *are* insterested in.
    # A lot of other programs still get installed, however, but disabling
    # them does not gain much at build time, and is a loooong list that is
    # difficult to maintain...

So, we also update that comment to explain why we still anyway disable
installation of 'date'.

Signed-off-by: Conrad Ratschan <conrad.ratschan@collins.com>
[yann.morin.1998@free.fr:
  - unconditionally disable installing date
  - extend comment and commit log to explain why we need
    --enable-no-install-program=date despite the existing
    --enable-install-program=ln,realpath
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>