Drop vestigial allowImportingTsExtensions from eng tsconfig

No longer needed after emitter-diff switched to .js import specifiers;
restores tsconfig.eng.json to match main.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: l0lawrence

tsconfig.eng.json

@@ -1,8 +1,5 @@
 {
   "extends": "./tsconfig.base.json",
-  "compilerOptions": {
-    "allowImportingTsExtensions": true
-  },
   "include": ["eng"],
   "exclude": ["eng/vitest.config.ts"]
 }