docs: kbuild: remove description of KBUILD_LDS_MODULE

Commit 596b0474

 ("kbuild: preprocess module linker script")
removes KBUILD_LDS_MODULE, yet the variable is still mentioned in
kbuild documentation. Remove the reference to the now-nonexistent
variable.

Signed-off-by: Sangmoon Kim <sangmoon.kim@samsung.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>