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

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-In principle, this memory allocator is roughly equivalent to Doug
-Lea's dlmalloc with fine-grained locking.
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-
-malloc:
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-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.
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-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.
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-