Remove hardcoded default parameter G1GC

[pucgenie]

Fixes #684 . G1 garbage collector is Java 17's default. Doesn't fix hardcoded vanilla-copied parameter handling.

[CLAassistant]

All committers have signed the CLA.

[MinetaS]

Old modpacks may not use Java 17.

https://github.com/FTBTeam/FTB-App/blob/86d919d5e912e15683e89c82186a6e93700d4261/subprocess/src/main/java/net/creeperhost/creeperlauncher/pack/InstanceLauncher.java#L455-L468

It will be better to add MOJANG_DEFAULT_ARGS flags before adding jvmArgs so that end users can override these flags.