initramfs: remove redundant dependency on BLK_DEV_INITRD

init/Kconfig includes usr/Kconfig inside the "if BLK_DEV_INITRD" ...
"endif" block:

    if BLK_DEV_INITRD

    source "usr/Kconfig"

    endif

Hence, all the defines in usr/Kconfig depend on BLK_DEV_INITRD.

Remove the redundant "depends on BLK_DEV_INITRD".

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Greg Thelen <gthelen@google.com>