remove stale document from malloc src directory

this was an unfinished draft document present since the initial
check-in, that was never intended to ship in its current form. remove
it as part of reorganizing for replacement of the allocator.

src/malloc/DESIGN

@@ -1,22 +0,0 @@
-
-
-In principle, this memory allocator is roughly equivalent to Doug
-Lea's dlmalloc with fine-grained locking.
-
-
-
-malloc:
-
-Uses a freelist binned by chunk size, with a bitmap to optimize
-searching for the smallest non-empty bin which can satisfy an
-allocation. If no free chunks are available, it creates a new chunk of
-the requested size and attempts to merge it with any existing free
-chunk immediately below the newly created chunk.
-
-Whether the chunk was obtained from a bin or newly created, it's
-likely to be larger than the requested allocation. malloc always
-finishes its work by passing the new chunk to realloc, which will
-split it into two chunks and free the tail portion.
-
-
-