DB/Spell: Delete no longer needed hack

By unholychick
DB/Spell: Do not let Ebon Plague stack from different casters

Closes #12314
Updates #1818
By unholychick
Update README.md
DB/SAI: Prevent endless loops with linked events
Revert bfc6044
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore into ghbb