Remove source-map dependency from terser bundle (#17067)

When updating terser to 4.1.0 in #17035 I inadvertently introduced a
dependency on the node source-map module.  Rebuilding after patching
our terser fork removed this:

emscripten-core/terser@b3ace3b

This happens to be available during testing because it one of our dev
decpendencies but we not include it in our end-user/production
dependencies.

This kind of error is hard to find since the `node_modules` for our test
setup is the same as the node_modules for emscripten itself.  It migth
be worth attempting to tease them appart if this kind of thing ever
shows up again.

Author: sbc100

third_party/terser/README.md

@@ -13,8 +13,5 @@ To make changes to this code please submit patches to
 https://github.com/emscripten-core/terser/ and then re-create this bundle
 using the following steps:
 
-  $ CI=1 npm run build
-  $ cp dist/bundle.min.js $EMSCRIPTEN_ROOT/third_party/terser/terser.js
-
-Despite its name, `dist/bundle.min.js` should not be minified (due to the `CI=1`
-in the command avove).
+  $ npx rollup -c
+  $ cp dist/bundle.js $EMSCRIPTEN_ROOT/third_party/terser/terser.js