[fuzz] Remove some differential fuzz targets (#4735)

* [fuzz] Remove some differential fuzz targets

The changes in #4515 do everything the `differential_spec` and
`differential_wasmi` fuzz target already do. These fuzz targets are now
redundant and this PR removes them. It also updates the fuzz
documentation slightly.

.github/workflows/main.yml

@@ -196,7 +196,8 @@ jobs:
       with:
         toolchain: nightly-2022-04-27
     - run: cargo install cargo-fuzz --vers "^0.11"
-    # Install OCaml packages necessary for 'differential_spec' fuzz target.
+    # Install the OCaml packages necessary for fuzz targets that use the
+    # `wasm-spec-interpreter`.
     - run: sudo apt install -y ocaml-nox ocamlbuild ocaml-findlib libzarith-ocaml-dev
     - run: cargo fetch
       working-directory: ./fuzz