Simplify last stand mixin

[jchung01]

Moves last stand fix into the LastStandEnchantmentsHandler mixin, removing the need of capturing the event handler. No more NPEs if it wasn't captured due to disabling the enchant in the config.
Fixes #534.

[what-the-diff]

PR Summary

• Removal of UTOpenBlocksEvents class
  The UTOpenBlocksEvents class from the mod.acgaming.universaltweaks.mods.openblocks package has been removed. This could lead to performance improvements if it was redundant.

• Deletion of UTConfigMixin class
  The UTConfigMixin class from the mod.acgaming.universaltweaks.mods.openblocks.mixin package has also been deleted. This might translate to better efficiency in the application if the class was no longer in use.

• Update to UTLastStandEnchantmentsHandlerMixin Class
  We've introduced two new methods, onHurt and ut$fixLastStand, in the UTLastStandEnchantmentsHandlerMixin class from the mod.acgaming.universaltweaks.mods.openblocks.mixin package. This could be helpful in managing events related to the 'last stand' enchantments more efficiently.

• Changes to the mixins.mods.openblocks.json File
  Alterations have been made to the mixins.mods.openblocks.json file. These modificiations will likely improve how we incorporate openblock modifications into our project.