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.