certs: use 'cmd' to hide openssl output in silent builds more simply

Commit 5d06ee20 ("modsign: hide openssl output in silent builds")
silenced the key generation log from openssl in silent builds.

Since commit 174a1dcc

 ("kbuild: sink stdout from cmd for silent
build"), the 'cmd' macro can handle it in a cleaner way.

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