certs: remove noisy messages while generating the signing key

When you run Kbuild with the parallel option -j, the messages from this
rule and others are interleaved, like follows:

    ###
      CC      arch/x86/mm/pat/set_memory.o
    ### Now generating an X.509 key pair to be used for signing modules.
    ###
    ### If this takes a long time, you might wish to run rngd in the
    ### background to keep the supply of entropy topped up.  It
      CC      arch/x86/events/intel/bts.o
      HDRTEST usr/include/linux/qnx4_fs.h
      CC      arch/x86/events/zhaoxin/core.o
    ### needs to be run as root, and uses a hardware random
    ### number generator if one is available.
      AR      init/built-in.a
    ###

On modern machines, it does not take a long time to generate the key.

Remove the ugly log messages.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>