initramfs: Remove redundant dependency of RD_ZSTD on BLK_DEV_INITRD

Commit be1859bd ("initramfs: remove redundant dependency on
BLK_DEV_INITRD") removed all redundant dependencies on BLK_DEV_INITRD,
but the recent addition of zstd support introduced a new one.

Fixes: a30d8a39 ("usr: Add support for zstd compressed initramfs")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>