Revert: "a" for "vim::AngleBrackets" (#20918)

The replacement "g" didn't seem to work for everyone.

Closes #20912
Updates #20104

Release Notes:

- vim: Restores `dia` to mean "delete in argument" instead of "delete
within angle brackets". To keep this in your own keymap use:
    ```
    {
"context": "vim_operator == a || vim_operator == i || vim_operator ==
cs",
        "use_layout_keys": true,
        "bindings": {
            "a": "vim::AngleBrackets"
        }
    }
    ```

assets/keymaps/vim.json

@@ -381,8 +381,7 @@
       "shift-b": "vim::CurlyBrackets",
       "<": "vim::AngleBrackets",
       ">": "vim::AngleBrackets",
-      "a": "vim::AngleBrackets",
-      "g": "vim::Argument"
+      "a": "vim::Argument"
     }
   },
   {