Fix InternalStore.removeOverride to call saveOverrides for state persistence (#412)

# Fix InternalStore.removeOverride to call saveOverrides for state
persistence

## Summary

Fixes bug where `InternalStore.removeOverride()` doesn't call
`saveOverrides()` and thus prevents state from being persisted. The
method was removing override values from the in-memory `localOverrides`
dictionary but not saving the changes to `StatsigUserDefaults`, causing
removed overrides to reappear after app restart.

**Root cause:** All other override methods (`overrideGate`,
`overrideConfig`, `overrideLayer`, `overrideParamStore`) follow the
pattern of calling `saveOverrides()` after modifying the
`localOverrides` dictionary, but `removeOverride` was missing this call.

**Fix:** Added `self?.saveOverrides()` call inside the async barrier
block in `removeOverride()` to match the existing pattern.

## Review & Testing Checklist for Human

- [ ] **Test override persistence across app restarts** - Verify that
when you remove an override, it stays removed after killing and
restarting the app (this was the core bug)
- [ ] **Run existing LocalOverridesSpec tests** - Ensure all override
removal tests in `Tests/StatsigTests/LocalOverridesSpec.swift` still
pass
- [ ] **Verify threading safety** - Confirm the `saveOverrides()` call
is correctly placed inside the `storeQueue.async(flags: .barrier)` block

---

### Diagram

```mermaid
%%{ init : { "theme" : "default" }}%%
graph TB
    API["Statsig.removeOverride()"]:::context
    Client["StatsigClient.removeOverride()"]:::context
    Store["InternalStore.removeOverride()"]:::major-edit
    SaveMethod["InternalStore.saveOverrides()"]:::context
    UserDefaults["StatsigUserDefaults"]:::context
    
    API --> Client
    Client --> Store
    Store --> SaveMethod
    SaveMethod --> UserDefaults
    
    Store -.-> |"NEW: Added missing call"| SaveMethod
    
    subgraph Legend
        L1["Major Edit"]:::major-edit
        L2["Minor Edit"]:::minor-edit  
        L3["Context/No Edit"]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF
```

### Notes

- **Risk level: Low** - Single line addition following established
pattern
- **Testing limitation:** Unable to run Swift tests locally due to
missing build tools, relying on CI
- **Pattern consistency:** Change makes `removeOverride` consistent with
all other override methods
- **Thread safety:** Change maintains existing threading model using
barrier queue

**Link to Devin run:**
https://app.devin.ai/sessions/571fff3bed7b4370956aa20c546971f1

**Requested by:** Andre Terron (@andre-statsig)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: andre-statsig

Sources/Statsig/InternalStore.swift

@@ -786,6 +786,7 @@ class InternalStore {
             self?.localOverrides[jsonDict: InternalStore.configsKey]?.removeValue(forKey: name)
             self?.localOverrides[jsonDict: InternalStore.layerConfigsKey]?.removeValue(forKey: name)
             self?.localOverrides[jsonDict: InternalStore.paramStoresKey]?.removeValue(forKey: name)
+            self?.saveOverrides()
         }
     }