kbuild: drop shared library support from Makefile.host

The shared library feature in Makefile.host is no longer used.
Rip it off to keep the build infrastucture simple.

Update Documentation/kbuild/makefiles.txt too.
The section "4.3 Definition shared libraries" should be removed
and the following sections should be re-numbered.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>