kbuild: remove command line interface LDFLAGS_MODULE from makefiles.txt

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>