remove GC_PTR_STORE_AND_DIRTY

turns out it's only necessary for MANUAL_VDB, which nix doesn't use. omitting
them gives a slight performance improvement on eval.

before:

  Benchmark 1: nix flakes search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      6.988 s ±  0.061 s    [User: 5.935 s, System: 0.845 s]
    Range (min … max):    6.865 s …  7.075 s    20 runs

  Benchmark 2: nix flakes eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     332.6 ms ±   3.9 ms    [User: 299.6 ms, System: 32.9 ms]
    Range (min … max):   328.1 ms … 339.1 ms    20 runs

  Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.681 s ±  0.049 s    [User: 2.382 s, System: 0.228 s]
    Range (min … max):    2.607 s …  2.776 s    20 runs

after:

  Benchmark 1: nix flakes search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      6.946 s ±  0.041 s    [User: 5.875 s, System: 0.835 s]
    Range (min … max):    6.834 s …  7.005 s    20 runs

  Benchmark 2: nix flakes eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     330.3 ms ±   2.5 ms    [User: 299.2 ms, System: 30.9 ms]
    Range (min … max):   327.5 ms … 337.7 ms    20 runs

  Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.671 s ±  0.035 s    [User: 2.370 s, System: 0.232 s]
    Range (min … max):    2.597 s …  2.749 s    20 runs

src/libexpr/eval.cc

@@ -879,7 +879,7 @@ Value * EvalState::allocValue()
     /* GC_NEXT is a convenience macro for accessing the first word of an object.
        Take the first list item, advance the list to the next item, and clear the next pointer. */
     void * p = *valueAllocCache;
-    GC_PTR_STORE_AND_DIRTY(&*valueAllocCache, GC_NEXT(p));
+    *valueAllocCache = GC_NEXT(p);
     GC_NEXT(p) = nullptr;
 
     nrValues++;