Undeprecate isNull

There's no good reason to deprecate it:
- For consistency reasons it should continue to exist, such that all
primitive types have a corresponding `builtins.is*` primop.
- There's no implementation cost to continuing to have this function
- It costs users time to try to migrate away from it, e.g.
  https://github.com/NixOS/nixpkgs/pull/219747 and https://github.com/NixOS/nixpkgs/pull/275548
- Using it can give easier-to-read code like `all isNull list`

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libexpr/primops.cc

@@ -438,9 +438,7 @@ static RegisterPrimOp primop_isNull({
     .doc = R"(
       Return `true` if *e* evaluates to `null`, and `false` otherwise.
 
-      > **Warning**
+      This is equivalent to `e == null`.
-      >
-      > This function is *deprecated*; just write `e == null` instead.
     )",
     .fun = prim_isNull,
 });