src: remove _third_party_main support

Since 7dead84, there is a more official alternative that is
tested and comes with a proper API, and since a6c57cc, the
`LoadEnvironment(env)` overload is deprecated, which is the closest
thing we can achieve to deprecating `_third_party_main` support.

Thus, we can now consider us able to remove `_third_party_main`
support.

Fixes: nodejs#24017
Refs: nodejs#30467
Refs: nodejs#32858

lib/internal/cli_table.js

@@ -11,8 +11,7 @@ const { getStringWidth } = require('internal/util/inspect');
 // The use of Unicode characters below is the only non-comment use of non-ASCII
 // Unicode characters in Node.js built-in modules. If they are ever removed or
 // rewritten with \u escapes, then a test will need to be (re-)added to Node.js
-// core to verify that Unicode characters work in built-ins. Otherwise,
-// consumers using Unicode in _third_party_main.js will run into problems.
+// core to verify that Unicode characters work in built-ins.
 // Refs: https://github.com/nodejs/node/issues/10673
 const tableChars = {
   /* eslint-disable node-core/non-ascii-character */

node.gyp

@@ -161,7 +161,6 @@
       'lib/internal/main/prof_process.js',
       'lib/internal/main/repl.js',
       'lib/internal/main/run_main_module.js',
-      'lib/internal/main/run_third_party_main.js',
       'lib/internal/main/worker_thread.js',
       'lib/internal/modules/run_main.js',
       'lib/internal/modules/package_json_reader.js',

src/node.cc

@@ -461,13 +461,6 @@ MaybeLocal<Value> StartExecution(Environment* env, StartExecutionCallback cb) {
     return scope.EscapeMaybe(cb(info));
   }
 
-  // To allow people to extend Node in different ways, this hook allows
-  // one to drop a file lib/_third_party_main.js into the build
-  // directory which will be executed instead of Node's normal loading.
-  if (NativeModuleEnv::Exists("_third_party_main")) {
-    return StartExecution(env, "internal/main/run_third_party_main");
-  }
-
   if (env->worker_context() != nullptr) {
     return StartExecution(env, "internal/main/worker_thread");
   }