Remove RegisterPrimOp constructor without support for documentation

The remaining constructor RegisterPrimOp::RegisterPrimOp(Info && info)
allows specifying the documentation in .args and .doc members of the
Info structure.

Commit 8ec1ba0210 removed all uses of the removed constructor in the
nix binary. Here, we remove the constructor completely as well as its
use in a plugin test. According to #8515, we didn't promis to maintain
compatibility with external plugins.

Fixes #8515

src/libexpr/primops.cc

@@ -4058,18 +4058,6 @@ static RegisterPrimOp primop_splitVersion({
 RegisterPrimOp::PrimOps * RegisterPrimOp::primOps;
 
 
-RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun)
-{
-    if (!primOps) primOps = new PrimOps;
-    primOps->push_back({
-        .name = name,
-        .args = {},
-        .arity = arity,
-        .fun = fun,
-    });
-}
-
-
 RegisterPrimOp::RegisterPrimOp(Info && info)
 {
     if (!primOps) primOps = new PrimOps;

src/libexpr/primops.hh

@@ -28,11 +28,6 @@ struct RegisterPrimOp
      * will get called during EvalState initialization, so there
      * may be primops not yet added and builtins is not yet sorted.
      */
-    RegisterPrimOp(
-        std::string name,
-        size_t arity,
-        PrimOpFun fun);
-
     RegisterPrimOp(Info && info);
 };
 

tests/plugins/plugintest.cc

@@ -21,4 +21,8 @@ static void prim_anotherNull (EvalState & state, const PosIdx pos, Value ** args
         v.mkBool(false);
 }
 
-static RegisterPrimOp rp("anotherNull", 0, prim_anotherNull);
+static RegisterPrimOp rp({
+    .name = "anotherNull",
+    .arity = 0,
+    .fun = prim_anotherNull,
+});