docs: Remove instruction topology

Instructions are stored in a contiguous array. So having an intrusive
linked list doesn't really make sense, Basic blocks will be responsible
for indexing into the instructions array.

Signed-off-by: Ronald Caesar <github43132@proton.me>

src/IR_DESIGN_DOC.md

@@ -93,10 +93,6 @@ typedef struct
     // Bitfields.
     uint16_t flags;
 
-    // The Topology (Intrusive Linked List)
-    uint32_t next;
-    uint32_t previous;
-
     // SSA versions
     // x3 = y1 + z2
     uint32_t definition; // x3