Tests/Fuzzing: Remove some obsolete experimental V8 flags

[kripken]

Those flags were removed in V8 as the features are no longer experimental. This
PR removes this noise from our logs:

Warning: unknown flag --experimental-wasm-gc.                                                                                                                                                 
Try --help for options                                                                                                                                                                        
Warning: unknown flag --experimental-wasm-typed-funcref.                                                                                                                                      
Try --help for options                                                                         
Warning: unknown flag --experimental-wasm-extended-const.                                                                                                                                     
Try --help for options                                                                         
Warning: unknown flag --wasm-final-types.                                                      
Try --help for options                                                                         

However, older versions of V8 may not work any more without those flags. I
always test on a recent one, and I'm not sure if we have a use for anything
else?

[aheejin]

I think it should be fine given that the only place V8_OPTS is used in fuzz_opt.py.