C API: Keep the structure flat

See https://github.com/NixOS/nix/pull/10329

src/libutil/c/nix_api_util_internal.h

@@ -1,49 +0,0 @@
-#ifndef NIX_API_UTIL_INTERNAL_H
-#define NIX_API_UTIL_INTERNAL_H
-
-#include <string>
-#include <optional>
-
-#include "error.hh"
-#include "nix_api_util.h"
-
-struct nix_c_context
-{
-    nix_err last_err_code = NIX_OK;
-    std::optional<std::string> last_err = {};
-    std::optional<nix::ErrorInfo> info = {};
-    std::string name = "";
-};
-
-nix_err nix_context_error(nix_c_context * context);
-
-/**
- * Internal use only.
- *
- * Helper to invoke nix_observe_string
- * @param context optional, the context to store errors in if this function
- * fails
- * @param str The string to observe
- * @param callback Called with the observed string.
- * @param user_data optional, arbitrary data, passed to the callback when it's called.
- * @return NIX_OK if there were no errors.
- * @see nix_observe_string
- */
-nix_err call_nix_observe_string(const std::string str, void * callback, void * user_data);
-
-#define NIXC_CATCH_ERRS \
-    catch (...) \
-    { \
-        return nix_context_error(context); \
-    } \
-    return NIX_OK;
-
-#define NIXC_CATCH_ERRS_RES(def) \
-    catch (...) \
-    { \
-        nix_context_error(context); \
-        return def; \
-    }
-#define NIXC_CATCH_ERRS_NULL NIXC_CATCH_ERRS_RES(nullptr)
-
-#endif // NIX_API_UTIL_INTERNAL_H