chore: remove MULTI_FILE_APPLY_DIFF experiment (RooCodeInc#10925)

* chore: remove MULTI_FILE_APPLY_DIFF experiment

Remove the 'Enable concurrent file edits' experimental feature that
allowed editing multiple files in a single apply_diff call.

- Remove multiFileApplyDiff from experiment types and config
- Delete MultiFileSearchReplaceDiffStrategy class and tests
- Delete MultiApplyDiffTool wrapper and tests
- Remove experiment-specific code paths in Task.ts, generateSystemPrompt.ts, and presentAssistantMessage.ts
- Remove special handling in ExperimentalSettings.tsx
- Remove translations from all 18 locale files

The existing MultiSearchReplaceDiffStrategy continues to handle
multiple SEARCH/REPLACE blocks within a single file.

* fix: remove unused EXPERIMENT_IDS/experiments import from Task.ts

Addresses review feedback: removes the unused imports from
src/core/task/Task.ts that were left over after removing the
MULTI_FILE_APPLY_DIFF experiment routing code.

Author: hannesrudolph