chore(core) Remove Feature::PowershellUtf8 (#15128)

## Summary
This feature has been enabled for powershell for a while now, let's get
rid of the logic

## Testing
- [x] Unit tests

codex-rs/core/config.schema.json

@@ -431,9 +431,6 @@
             "plugins": {
               "type": "boolean"
             },
-            "powershell_utf8": {
-              "type": "boolean"
-            },
             "prevent_idle_sleep": {
               "type": "boolean"
             },
@@ -2040,9 +2037,6 @@
         "plugins": {
           "type": "boolean"
         },
-        "powershell_utf8": {
-          "type": "boolean"
-        },
         "prevent_idle_sleep": {
           "type": "boolean"
         },