kbuild: remove unused OBJSIZE

The "size" tool has been solely used by s390 to enforce .bss section usage
restrictions in early startup code. Since commit 980d5f9a ("s390/boot:
enable .bss section for compressed kernel") and commit 2e83e0eb
("s390: clean .bss before running uncompressed kernel") these restrictions
have been lifted for the decompressor and uncompressed kernel and the
size tool is now unused.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>