Move old note in proof mode docs.

This note is obsolete due to iris/iris!640

docs/proof_mode.md

@@ -332,12 +332,7 @@ _introduction patterns_:
   to destruct nested (separating) conjunctions.
 - `[ipat1|ipat2]` : disjunction elimination.
 - `[]` : false elimination.
-- `%H` : move the hypothesis to the pure Coq context, and name it `H`. Support
-  for the `%H` introduction pattern requires an implementation of the hook
-  `string_to_ident`. Without an implementation of this hook, the `%H` pattern
-  will fail. We provide an implementation of the hook using Ltac2, which works
-  with Coq 8.11 and later, and can be installed with opam; see
-  [iris/string-ident](https://gitlab.mpi-sws.org/iris/string-ident) for details.
+- `%H` : move the hypothesis to the pure Coq context, and name it `H`.
 - `%` : move the hypothesis to the pure Coq context (anonymously). Note that if
   `%` is followed by an identifier, and not another token, a space is needed
   to disambiguate from `%H` above.