libutil: delete unused boost context cruft

This was from before we got rid of the boost coroutines. Now we don't
need any of this code.

Change-Id: Ief8e8ebc184f02f48e30cb253a66b540faa56329

src/libutil/serialise.hh

@@ -9,8 +9,6 @@
 #include "types.hh"
 #include "file-descriptor.hh"
 
-namespace boost::context { struct stack_context; }
-
 namespace nix {
 
 
@@ -612,23 +610,6 @@ struct FramedSink : nix::BufferedSink
     };
 };
 
-/**
- * Stack allocation strategy for sinkToSource.
- * Mutable to avoid a boehm gc dependency in libutil.
- *
- * boost::context doesn't provide a virtual class, so we define our own.
- */
-struct StackAllocator {
-    virtual boost::context::stack_context allocate() = 0;
-    virtual void deallocate(boost::context::stack_context sctx) = 0;
-
-    /**
-     * The stack allocator to use in sinkToSource and potentially elsewhere.
-     * It is reassigned by the initGC() method in libexpr.
-     */
-    static StackAllocator *defaultAllocator;
-};
-
 /* Disabling GC when entering a coroutine (without the boehm patch).
    mutable to avoid boehm gc dependency in libutil.
  */