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: NMasahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/kbuild.txt

@@ -50,10 +50,6 @@ LDFLAGS_MODULE
 --------------------------------------------------
 Additional options used for $(LD) when linking modules.
 
-LDFLAGS_vmlinux
---------------------------------------------------
-Additional options passed to final link of vmlinux.
-
 KBUILD_VERBOSE
 --------------------------------------------------
 Set the kbuild verbosity. Can be assigned same values as "V=...".