chore(core): remove experimental wasm bigint flag (#7790)

This removes the experimental bigint flag as it is enabled by default
now and is no longer necessary.

core/runtime.rs

@@ -157,7 +157,6 @@ pub unsafe fn v8_init() {
     "--wasm-test-streaming".to_string(),
     "--no-wasm-async-compilation".to_string(),
     "--harmony-top-level-await".to_string(),
-    "--experimental-wasm-bigint".to_string(),
   ];
   v8::V8::set_flags_from_command_line(argv);
 }