certs: move the 'depends on' to the choice of module signing keys

When the condition "MODULE_SIG || (IMA_APPRAISE_MODSIG && MODULES)"
is unmet, you cannot choose anything in the choice, but the choice
menu is still displayed in the menuconfig etc.

Move the 'depends on' to the choice to hide the meaningless menu.

Also delete the redundant 'default'. In a choice, the first entry is
the default.

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