kbuild: remove command line interface LDFLAGS_MODULE from makefiles.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.

LDFLAGS_MODULE is a command line interface, so it should be dropped
from makefiles.txt.

Some lines below in this file, it is clearly explained that
KBUILD_LDFLAGS_MODULE is the right one for the internal use:

    KBUILD_LDFLAGS_MODULE   Options for $(LD) when linking modules

        $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
        used when linking modules. This is often a linker script.
        From commandline LDFLAGS_MODULE shall be used (see kbuild.txt).

Then, kbuild.txt explains LDFLAGS_MODULE, like follows:

    LDFLAGS_MODULE
    --------------------------------------------------
    Additional options used for $(LD) when linking modules.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/makefiles.txt

@@ -837,12 +837,6 @@ When kbuild executes, the following steps are followed (roughly):
 	Note: ldflags-y can be used to further customise
 	the flags used. See chapter 3.7.
 
-    LDFLAGS_MODULE	Options for $(LD) when linking modules
-
-	LDFLAGS_MODULE is used to set specific flags for $(LD) when
-	linking the .ko files used for modules.
-	Default is "-r", for relocatable output.
-
     LDFLAGS_vmlinux	Options for $(LD) when linking vmlinux
 
 	LDFLAGS_vmlinux is used to specify additional flags to pass to