kbuild: remove append operation on cmd_ld_ko_o

The append operation was introduced in
commit b1a1a1a0 ("kbuild: lto: postpone objtool")
when the command was created from two parts.
In commit 850ded46 ("kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG")
however the first part was removed again, making the append operation
unnecessary.

To keep this command definition aligned with all other command
definitions, remove the append again.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>