initramfs: make initramfs compression choice non-optional

Currently, the choice of the initramfs compression mode is too complex
because users are allowed to not specify the compression mode at all.

I think it makes more sense to require users to choose the compression
mode explicitly, and delete the fallback defaults of INITRAMFS_COMPRESSION.

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