kbuild: remove internally used LDFLAGS_vmlinux from kbuild.txt

Documentation/kbuild/makefiles.txt lists variables used in Makefile
whereas Documentation/kbuild/kbuild.txt describes user assignable
parameters given via environments or the command line.

The top Makefile and arch/*/Makefile accumulate proper linker flags to
LDFLAGS_vmlinux.  So, users can not override it from the command line.
Generally, per-file options are not supposed to be user-assignable.
Remove the misleading entry from kbuild.txt.

If we need a way to append user-specific flags for linking the kernel,
LDFLAGS_KERNEL would be a consistent choice because we already expose
LDFLAGS_MODULE counter-part to users.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>