Deprecate -sMAYBE_WASM2JS settings (#23970)

Users of this settings will now see:

```
emcc: warning: MAYBE_WASM2JS is deprecated (no known users). Please open a bug if you have a continuing need for this setting [-Wdeprecated]
```

See #23969

Author: sbc100

test/test_core.py

@@ -8388,7 +8388,7 @@ def test_maybe_wasm2js(self):
       self.skipTest('redundant to test wasm2js in wasm2js* mode')
     self.set_setting('MAYBE_WASM2JS')
     # see that running as wasm works
-    self.do_core_test('test_hello_world.c')
+    self.do_core_test('test_hello_world.c', emcc_args=['-Wno-deprecated'])
     # run wasm2js, bundle the code, and use the wasm2js path
     cmd = [PYTHON, path_from_root('tools/maybe_wasm2js.py'), 'test_hello_world.js', 'test_hello_world.wasm']
     if self.is_optimizing():

tools/settings.py

@@ -126,6 +126,7 @@
     'LEGALIZE_JS_FFI': 'to disable JS type legalization use `-sWASM_BIGINT` or `-sSTANDALONE_WASM`',
     'ASYNCIFY_EXPORTS': 'please use JSPI_EXPORTS instead',
     'EMULATE_FUNCTION_POINTER_CASTS': 'lack of usage',
+    'MAYBE_WASM2JS': 'lack of usage',
 }
 
 # Settings that don't need to be externalized when serializing to json because they