Optimization -O1, -O2, ... get rid of the environment variables handling in file.js

Basically, the code related to the environment variables handling

```js
var ENV = {
};

var getExecutableName = () => {
return thisProgram || './this.program';
};
var getEnvStrings = () => {
if (!getEnvStrings.strings) {
// Default values.
// Browser language detection #8751
var lang = ((typeof navigator == 'object' && navigator.languages && navigator.languages[0]) || 'C').replace('-', '_') + '.UTF-8';
var env = {
'USER': 'web_user',
'LOGNAME': 'web_user',
'PATH': '/',
'PWD': '/',
'HOME': '/home/web_user',
'LANG': lang,
'_': getExecutableName()
};
// Apply the user-provided values, if any.
for (var x in ENV) {
// x is a key in ENV; if ENV[x] is undefined, that means it was
// explicitly set to be so. We allow user code to do that to
// force variables with default values to remain unset.
if (ENV[x] === undefined) delete env[x];
else env[x] = ENV[x];
}
var strings = [];
for (var x in env) {
strings.push(`${x}=${env[x]}`);
}
getEnvStrings.strings = strings;
}
return getEnvStrings.strings;
};

```
It is removed when compiling with optimizations. Setting up a --pre-js extending the environment leads to an error of ENV not declared.

**Version of emscripten/emsdk:**
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.46 (19607820c447a13fd8d0b7680c56148427d6e1b8)
clang version 18.0.0 (https://github.com/llvm/llvm-project 75501f53624de92aafce2f1da698b249a7293dc7)
Target: wasm32-unknown-emscripten
Thread model: posix