tsconfig: Remove *.js exclusion.

This improves the editor experience for *.js, and there’s not a big
performance impact anymore.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tsconfig.json

@@ -46,8 +46,5 @@
         "static/webpack-bundles",
         "var",
         "zulip-py3-venv",
-
-        // Even though allowJs defaults to false, typescript-eslint forces it to true.
-        "**/*.js",
     ],
 }