documentation: remove smart-config.txt

As requested by Sam Ravnborg: Remove Documentation/smart-config.txt.

It is outdated and has been (functionally) replaced by
Documentation/kbuild/*.txt.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>