memory.txt: remove stray information

Andi removed some outedated documentation from Documentation/memory.txt
back in 2009 by commit 3b2b9a8 ("Documentation/memory.txt: remove
some very outdated recommendations"), but the resulting document is not
in a nice shape either.

It seems to me like we are not losing anything by completely removing the
file now.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>