kbuild: drop $(size_append) from cmd_zstd

The appended file size is only used by the decompressors, which some
architectures support.

As the comment "zstd22 is used for kernel compression" says, cmd_zstd22
is used in arch/{mips,s390,x86}/boot/compressed/Makefile.

On the other hand, there is no good reason to append the file size to
cmd_zstd since it is used for other purposes.

Actually cmd_zstd is only used in usr/Makefile, where the appended file
size is rather harmful.

The initramfs with its file size appended is considered as corrupted
data, so commit 65e00e04 ("initramfs: refactor the initramfs build
rules") added 'override size_append := :' to make it no-op.

As a conclusion, this $(size_append) should not exist here.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>