[Discover] Remove allow js setting (#148286)

## Summary

Closes #144283

This PR removes `allowJS` setting.

src/plugins/discover/tsconfig.json

@@ -1,12 +1,7 @@
 {
   "extends": "../../../tsconfig.base.json",
   "compilerOptions": {
-    "outDir": "target/types",
-    // there is still a decent amount of JS in this plugin and we are taking
-    // advantage of the fact that TS doesn't know the types of that code and
-    // gives us `any`. Once that code is converted to .ts we can remove this
-    // and allow TS to infer types from any JS file imported.
-    "allowJs": false
+    "outDir": "target/types"
   },
   "include": ["common/**/*", "public/**/*", "server/**/*", "../../../typings/**/*", ".storybook/**/*"],
   "kbn_references": [