Alias traceable_allocator to std::allocator when building without GC

This allows us to get rid of a bunch of #ifdefs.

src/libexpr/eval.cc

@@ -99,11 +99,7 @@ static const char * makeImmutableString(std::string_view s)
 
 RootValue allocRootValue(Value * v)
 {
-#if HAVE_BOEHMGC
     return std::allocate_shared<Value *>(traceable_allocator<Value *>(), v);
-#else
-    return std::make_shared<Value *>(v);
-#endif
 }
 
 // Pretty print types for assertion errors