chore: remove unused import

This import must be removed to prevent a compile error introduced in
0.14.0-dev.1409+6d2945f1f that prevents decls and fields from having the
same name.

src/ThreadHeapMap.zig

@@ -118,4 +118,3 @@ const Allocator = std.mem.Allocator;
 
 const Heap = @import("Heap.zig");
 const log = @import("log");
-const list = @import("list.zig");